A bad news for you, but I too have problem with your nomination on Mega Venusaur.
Apart from the fact that you have nominated already niche Pokémon for B-, I don't see a reason why you would believe Mega Venusaur has similar level of viability as things listed in such rank. Majority of them have strengths that often outshines their weakness. If you run Mega Venusaur, it means you are dedicating a Mega slot to check specific threats and that doesn't sound too appealing in the first place.
Let's go over your points. Yes, it does wall Arceus-Fairy, Xerneas, and Primal Kyogre with its typing. SpDef Pdon? It 2HKOs mega venusaur with any moves after Stealth Rock and you are forced to stay there and press Synthesis that has only 8 PP. Chances are, if one switches mvenu into offensive pdon with a thought that it is spdef and you can stall with Synthesis, you can be hit by full-powered Overheat which does upward 81% to max spdef mvenu or give a free turn for something like Swords Dance. Support Arceus, yes, mvenu checks them. Non ZMove Marshadow? If you run spdef it 2HKOs you with Spectral Thief after Stealth Rock. Celesteela is one of the worst declining Pokémon at this moment and I do not believe it is a good example to make a comparison with.
Until now mvenu seems okay. But here is the next issue; if mvenu runs maxdef, it is 2hkod by Origin Pulse after minimal chip and groundceus Judgment does upwards 42% meaning you have to spam Synthesis. If you run max spdef to alleviate these issues, that is the point where you worsen mvenu's ability to handle defensive Pdon and let it get 2HKOd by Marshadow after Stealth Rock.
You still end up with this much shaky wall that handles only couple specific threats when you dedicate a Mega slot. This already leaves us with a question related with opportunity cost. Also you also have mentioned utility moves but in reality you can't fit them all in one slot. First off you will obviously need Synthesis and Leech Seed. You already have two moveslots left when you theoretically wish to fit Sludge Bomb / Knock Off / Earthquake / Sleep Powder / Roar / Giga Drain.
Roar has to fit here; otherwise you will get straight up steamrolled by just about every Substitute users which are becoming increasingly common (and again, you should know this if you have been experimenting in the ladder). mmence / arceus / groundceus and pogre when you lack giga drain. That leaves you one slot. Knock Off is just awful because every switch-in to mvenu barring ho-oh and yveltal have irremovable items, Eathquake doesn't do jack outides hitting mega gengar (and you still lose to wis + hex), Sleep Powder is just bad with aforementioned reasons with Substitute users, and Giga Drain has like no coverage - not to mention pogre takes upwards 30% after a single Calm Mind and 2HKOs you back with Origin Pulse.
This much flaws come from the opportunity risk of handing over Mega Stone to mvenu. I seriously doubt that it is worth to dedicate a mega slot to a pokemon that has this much inconsistency issues as well as moveslot syndrome.
This statement is invalid at this point. Counter is when you can deal with a mon in all circumstances but there are clearly more than few occasions where mvenu fails to even check the threats it desires to switch into.
Giratina-A and Ho-Oh are just nightmares to deal with, and Chansey barely carries Seismic Toss when employed in stall teams and thus has no reason to switch into or stay in against mvenu. Mega Sableye is also a hard stop if you don't run Sludge Bomb and running it causes a loss of another utility move.
Another serious problem with this Pokémon is that you have to build around it when Mega Venusaur does very little in return for its team outsides seeding a switch-in and occasionally using sleep powder (which means you forgo sludge bomb / giga drain and become more passive). What I can say the better alternative is Ferrothorn - it doesn't check Xerneas and Marshadow but it is one of the three viable Spikes setter in the tier, checks CM ogre if carries Power Whip.
This much flaw exists with mvenu when you use your mega slot for it. The mega mon you build around loses to extremely common threats: pdon, necrozma-dm / any ultra necrozma, yveltal, mgar, mmence, sub zyg-c, ho-oh, subcm ogre, and rayquaza. I just went through current VR down for couple seconds and it turns out this many threats can take advantage of mvenu. That is not to mention it is also prone to Gothitelle trapping. I personally think mvenu has a similar level of viability as mega slowbro - which is currently unranked - because of the very few specific niche it has in exchange of countless, crippling flaws. You might say the "boons" cannot be ignored, but the so-called-unique role mvenu compresses is hold back due to it adding more defensive flaws than it covers, meaning it will entirely depend on its teammates to make a defensive team together and thus cannot afford to fit in BO - again, it has to be built around.
I'll address all of these issues that you present, some valid and others not so much. First off, there exists a mixed bulk spread (which I won't give away here for the purposes of hiding my teams) that deals with both Marshadow and Xerneas. Also the moveset should always include GK, as it is main way of pressuring a good portion of the meta, any mon that is weak to it will take a substantial amount, while anything neutral will not be allowed to set up for free in front of venu. Sludge is completely passable as the things it hits are already beaten etc. I mention the last moveslot as changable depending on the team, because altho there may be sub users, the primary ones that I have seen in tour play (which is where I would be using this team) are mmence, arceus, zygarde and occasionally yveltal, where you would have to run some sort of mmence counter on your team, (rockceus or groundceus are preffered here) and zygard/arceus' sub is broken by GK.
Another issue you bring up is the Mega Evolution. In my opinion Mega evolution since USM has become a significantly less important aspect of teambuilding, as while Gengar is very potent it is not super splashable and MMence is good but its roles can often be found in other slots anyway. On many offense and Balance teams I have found myself forgoing a mega altogether to pick up some other threats, for example PDon/DuskMane/Marshadow/Yveltal/Xerneas/Arceus is a team comp that is somewhat common and very much viable which forgoes a mega altogether.
Now for your next issue, which deals with its viability against stall, I shall give you this replay which shows you the reason why its very good.
https://replay.pokemonshowdown.com/gen7ubers-697364927
So if you watch the entire replay, (this was a test match between myself and hyw for smogexhi, hyw being the best stall player of usm), and you see the momentum of the battle is shifted in his favor as he manages to slowly chip away at all my mons, which lack any sort of recovery, and the only mon he really allows to get weakened is Bliss. But then, after a couple times when Venu comes in, it gets the Leech on blissey and is able to heal both Xern and Yveltal into a range to take hits from Ho-oh. On top of this, Venu is able to consistently come in on a number of mons which include Defog Ho-oh, Lugia, and he lacks a true way of being able to kill it at any point in the game outside of PP Stall. As such it allows for a reliable pivot should I ever be presented in a bad situation, i.e. I just choice locked myself into Aroma on Xern or something similar. And importantly this game, It was able to PP Stall the Ho-oh out of Sacred Fires (many times Defog Ho-oh will forgo BB on stall) and made Arceus Ground a nonfactor. Although you could claim a similar thing be done by other mons, this isn't exactly true, as it is able to be Toxic Immune while maintaining a neutrality to Fire and providing the team with Leech Support. If you believe this to be niche, then so be it, but a similar situation applies in many stall/bulky matchups as long as you have some sort of breakers within the team. Not to mention, in this game I had not brought any anti-stall tech (I was experimenting with HP Fire, which I later deemed is a useless move), as if Venu was carrying Toxic or Knock, the game would have been much easier.
Now this is where I cede some points as to its weaknesses, obviously you aren't always going to get matched into no flying stab hooh or no flying stab yveltal, and those mons can generally come in for free on Venu, barring Toxic/Leech chip. However, it should also be noted that MVenu balance provides great reactive play with it being easy to slot in answers to pretty much everything within a team with MVenu barring maybe Duskmane, and it relies on you to play the game well and win 50/50s. Of course its weaknesses are glaring as a lot of common meta mons do beat it, like SD PDon, Ekiller, Hooh, Offensive Yveltal, Ray, Arc Flying, Wisp Hex Gar, mmence and ofc DM, however it should also be noted how many of those same mons are checked by Arceus Ground, and then you just need a couple of other slots to cover the remaining weaknesses. I don't mean to downplay MVenu's weaknesses in anyway, I recognize it is a mon only suited to a specific teamcomposition, however I feel that team composition is a very viable G1 team in a bo3 (in fact I have won many of my bo3s in ubers ssnl by opening this team against ppl like Edgar and Arifeen).
Now finally to try to make this as nonbiased as possible, let's compare it to Arceus Dragon which is in the same tier. Much like MVenu, running Arceus Dragon has a high opporunity cost since it is taking up an Arceus form (arguably more important than a mega, but lets just say they are of comparable value) Arceus Dragon will have to run Recover and Judge and the last two moves are there to help in specific matchups, and depending on what is chosen, it will lose to a similar amount of mons within the top of the via rankings. From what I have seen Arceus Dragon is normally running Defog and either of the status moves, and I assume that those 3 moves are the viable other 2 slots. Regardless of what it picks, it will lose to defensive Pogre Xerneas Magearna and Gothitelle, while depending on the set it can lose to variants of DM, Hooh, Arceus Fairy, Ferrothorn and Spdef Don. Further beyond this, the threats it is supposed to beat, can chose specific tech options to cripple it. The threats that each of these mons check are of similar, each one having a multitude of counters, I would say Arc-Drag is even more passive, however also has Defog for some more team support. If you notice, the situation is similar, both of these mons check a subset of the top tier, Arc Drag specifically offensive Pdons, mixed Ogre, MMence etc, and doesnt straight lose to stuff like Zygarde and Marsh, offering a soft check to them. Venu I've gone over already, but both of these mons also remain very passive vs. another portion of the meta. Then each provides valuable team support through status, potential healing, Defog, Toxic Immunity etc.
TL;DR
When making this comparison, I think it is fair to rank MVenu alongside Arceus Dragon, because they are mons of a similar calibur used in a similar playstyle. I would even argue that MVenu is better but that is perhaps too ambitious and part of my personal bias. So in general, I think it definitely has a lot of flaws, but also a lot of things going for it.