There's a very limited number of Defog / Rapid Spin users and they all imply further support themselves because Ho-Oh transforms games (and thus your teambuilding) into giant crapshoot of an SR war. Excadrill p much implies TTar which means you've got a massive Kyogre weakness to work with and only 3 slots left. Defog Arceus need a cleric cause everything runs Toxic and you are banking on it to remove SR, oh and there's that tag weakness. Cloyster is cool but adds absolutely nothing to your team besides spinning and needs to be on a team that's offensive enough to not ask for that more than once. Tina-o has a host of weaknesses that has hurt its viability this gen and won't be Defogging on Dialga. Scizor eats up your mega slot, loses to Dialga Fire Blast, and can't really be used to pursuit Gar anymore since dbond means you lose your anti SR measure. Plus, it's not terribly good at finding free turns regularly enough to fight off SR in the first place. Basically, the difference between a non-Ho-Oh team with anti-hazard support and one with it is that the Ho-Oh team is utterly reliant on that support and needs to give support FOR THE SUPPORT so that it doesn't fail during a game.
Uh you can easily cover Kyogre with those remaining three slots. Drill and Ho-Oh work very well together anyway since can Ho-Oh can burn bulky Grounds and Waters for Drill while Drill resists Rocks and Electrics. If you decide to run TTar that also means you have Pursuit support, granted its not but so reliable but it does give you more freedom in your choice of Kyogre check since you can more safely run stuff like Arceus-Grass/Water. Palkia is always a good pick as well.
You don't necessarily need a cleric if you're smart about picking your Arceus form. Poison Arceus doesn't necessarily need a Defog user since its immune to Toxic (and Ho-Oh gives you a burn absorber) and Ghost Arceus is immune to Shadow Tag which gives you a lot more freedom in playing with it.
I get that Cloyster only works on HO but that doesn't mean its not a bad option.
Tina-O does have issues but there are enough things it can come in on (like most Grounds and things like that) to where it's not a terrible option and Ho-Oh can always be used as a switch into Fairies/LO Yveltals after Giratina-O has done its Defog duty. It's definitely not optimal but its not useless.
I won't argue Scizor since Scizor isn't the most reliable Defogger anyway, even on teams without Ho-Oh.
The reason why SR becomes so important vs teams with Ho-Oh is because Ho-Oh is such a good Pokemon that not keeping it up means it will tear your face off and slow cook it over an open fire. It is more dependent on anti-hazard support than most other Pokemon but the sheer amount of power, bulk, and utility Ho-Oh brings to the table makes the support worth it. It is also worth noting that a lot of team archetypes that Ho-Oh fits nicely on (stall, balance, Sand, bulky offense maybe) are teams that would normally use some kind of anti-hazard support even without a Ho-Oh. The fact that SR becomes hugely important when facing a Ho-Oh team is much more a testament to how good of a Pokemon it actually is since it definitely puts you under more pressure to keep it up then if, say, you were facing a Charizard.
Ho-Oh is one of the Pokemon that defines the metagame and putting it anywhere lower than A is laughable. Stop acting like having Stealth Rock on your team automatically means you're safe from Ho-Oh because that just isn't true.
As for the other things you mentioned:
Darkrai - It shouldn't be below A/A-. It still fulfills the (very important) offensive niche of checking CM Wisp Arcs with its Speed and Dark Void which is something Yveltal can't do at all, and Nasty Plot makes it a much more efficient cleaner. It is also much better at pressuring defensive Fairy checks due to access to Sludge Bomb which Yveltal can struggle against. Yeah Keys are annoying but Yveltal hates them too so thats not really a point against Darkrai (and before you claim Taunt, Darkrai can do that too).
Klefki - It's fine at A. It is a great Pokemon but it has some very noticeable flaws, mainly its lack of offensive presence and inability to really be a Dragon check because all the Uber dragons have ways to smash it. And as you said it doesn't jive well with Defog support which can hurt its ability to contribute to a match if you need such support on your team.
Deoxys-A - Disagree strongly, Deoxys-A is still a nightmarish offensive threat and it does not require much in the way of actual support.
Deoxys-S - I wouldn't be opposed to making this A-/B+ since all it does is set Spikes and die. It's still very much something you need to be ready for but its not as good as it used to be.
Zekrom - I somewhat disagree since Zekrom is pretty much the reason why Ground/Fairy is really popular on a lot of teams, and getting surprised by a non-Choiced set (Tailwind Zekrom is gnarly) can be absolutely devastating. Not too opposed to A- since people do prepare a lot more for it now, but it shouldn't be anywhere lower than that and it is still a solid Pokemon in the metagame.
Arceus-Electric - Not necessarily opposed, but not necessarily for since it sucks as a support forme, it does have a few archetypes it matches up poorly against (mainly talking Sand teams with Excadrill and stall), and it struggles hard against the things that do check it.
Gothitelle - I think S is a bit of a stretch but A seems appropriate just because of its ability to absolutely shred teams that give it an opening (of which there are quite a few). I'm not too opposed to S I guess but you oughta post an actual argument for it though :P
Dialga - I disagree, I made a post about it awhile ago and I don't really think anything has changed.
Arceus-Fairy - It kind of does need Pursuit because Gengar just wrecks this mon, also lacks the ability to be a cleric unlike its other Fairy friends Xerneas and Sylveon which hurts its defensive utility. Cool CM forme but it does get crapped on by the usual CM Arc checks (SDef Ogre, Ho-Oh, Toxic) and does face competition from GeoXern. So yeah I guess I disagree, being the best Yveltal check is a good plus though.
Arceus-Ground - SD Groundceus is very good and it does check Eleceus way better than Groudon or Lando so I kinda disagree.
Bronzong - Agree with C+, I like this mon but it does have a lot of issues in the current meta and its huge Gothitelle bait.
Forretress - Agreed.
Scolipede - I mostly agree, Scolipede's one big flaw as a suicide lead is that it can't SR so something else on your team has to handle that, but otherwise its cool and beating Deo leads is very nice. Loses very hard to Defog Ghosts and Scizor though.
Kyurem-W - I disagree since Thugly Duckling did make a pretty good team with it not too long ago. It's still really hard to switch in on, great mon for punishing Grounds, Klefki isn't safe at all due to Fusion Flare/EP both smashing it to pieces, and Ice Beam is quite spammable.
Charizard - Charizard is highly overrated and faces huge competition (read: mostly outclassed save a few things) from Blaziken (if X) and Ho-Oh (if Y), it is absolutely not B-rank material. I made a huge post about this already and I'm not going to repeat myself. Charizard fits the C-rank description perfectly anyhow.
Diancie - Disagree, Diancie has a lot of weaknesses and is still a relatively new mon, give it some more time.
TinaA - Agree
Arceus-Fire - Agree
Abomasnow - Agree
Webbers - I'm not the one you need to convince here (B+ is way too high though, I think B/B- is better)
Also I'm not adding Rhyperior since it was rejected for an analysis (as was Rhydon).
