Not gonna argue super hard or anything, but just some clarifications. (And on "there aren't many reasons to run this over other stuff in the meta", doesn't that apply to... literally everything in both ranks to start with?)
Zam: You forget the other niche of Zam is outspeeding MMX/Y, which lets it safely revenge KO roughly half the MMX/Y sets without resorting to Scarf or get momentum by forcing them out. (Zam's Moongeist 2HKOs MMY with hazards up, neutral nature, and no item/ability/terrain/move boosts, for example.)
It also makes it a slightly better Tail Glow user since it has to worry less than normal about being outsped. Phera still beats it by literally one point of base speed though, which sucks.
There's also the niche of naturally outrunning Scarf positive speed Hoopa-U. You'd have to run something weird like Bug Buzz to take full advantage of that though.
Heatran: It can't take V-Creates well, no, but that's all Steel-types anyway, FF aside. First big niche is it can self Imposter-proof or be easily Imposter-proofed thanks to its weakness. And while 130 special attack isn't impressive, it's good enough for some boosting sets or at least being less passive than others like Registeel.
Defensive niche though, double Fairy and double Ice resistance make it a good -ate check before you even consider ability. Double Grass, double Steel, and double Bug resistance might also have some value too, though I've not explored those avenues. Though running a calc, it has some promise as a Kartana check as well.
+6 252 Atk Kartana Sunsteel Strike vs. 252 HP / 252 Def Heatran: 159-187 (41.1 - 48.4%) -- guaranteed 3HKO
252 SpA Flash Fire Heatran Magma Storm vs. 252 HP / 252 SpD Kartana: 1432-1692 (444.7 - 525.4%) -- guaranteed OHKO
Might be a good mon who needs to compress a Diancie, Kyurem, and Kartana counter into one role? It can straight up tank and OHKO two of them and deal some good damage to the other. It probably also matches well against Xerneas and Sceptile too. Really, Levi Tran was actually a meta mon, probably mid-to-high VR if that had been a thing at the time, until Thousand Arrows became a thing.
Absol (Regular): Mainly to block non-Prank Spore, Glare, Whirlwind, Topsy, etc. Again, it's super niche and probably something you'd use when prepping a team for a particular opponent in a tournament lest the ladder meta really favored it.
Fur Coat Palk: Mainly if you need to check sets like this.
252+ Atk Choice Band Tinted Lens Blaziken-Mega V-create vs. 252 HP / 252 Def Palkia: 222-262 (57.8 - 68.2%) -- guaranteed 2HKO
252+ Atk Choice Band Tinted Lens Blaziken-Mega V-create vs. 252 HP / 252 Def Fur Coat Palkia: 110-132 (28.6 - 34.3%) -- 3.1% chance to 3HKO
Compare to Ogre.
252+ Atk Choice Band Tinted Lens Blaziken-Mega V-create vs. 252 HP / 252 Def Fur Coat Kyogre-Primal: 238-282 (58.9 - 69.8%) -- guaranteed 2HKO
Also worth noting...
252 Atk Groudon-Primal Precipice Blades vs. 252 HP / 252 Def Palkia: 198-234 (51.5 - 60.9%) -- guaranteed 2HKO
252 Atk Groudon-Primal Precipice Blades vs. 252 HP / 252 Def Fur Coat Palkia: 100-118 (26 - 30.7%) -- guaranteed 4HKO
Mane: Specs Ice Beam 2HKOs Giratina after 10-16% chip damage. Mane also becomes a fast breaker with E.Terrain support.
252 SpA Choice Specs Galvanize Manectric-Mega Boomburst vs. 252 HP / 252 SpD Giratina in Electric Terrain: 221-261 (43.8 - 51.7%) -- 10.9% chance to 2HKO
I mean, that's not an amazing number, but that is a resisted hit on one of the bulkiest walls in the meta. Either way, if it weren't for Ground-types, Specs Mane would literally not need any moves for coverage on E.Terrain teams.
Also worth noting non-specs sets can opt to run Icinium-Z, which technically 2HKOs Giratina if you Ice Beam and then Z-Ice Beam. Zice Beam also OHKOs Zygarde too. Really, the big threat Mane has to worry about is Primal Groudon since it can't OHKO it anymore with Steam Eruption. It's only chance, without teammates, is for an Earth Power on it switching in followed by a Z-Earth Power. Or somethinking funky/memish like Entrainment or Gastro Acid on the switch followed by Steam Eruption.
OM! Sheer Force T-Bolt is a little weaker than Zap Cannon. Not by much, but it's there. The real thing though is you miss out on the guaranteed paralysis which makes switching into Manectric a pain for anything not immune. Even if your opponent is addicted to slow pivoting, paralyzing their walls is usually a good play anyway since the 25% chance to not move is more and more likely to dick them over as the match continues. Also being able to run Blizzard also compensates no Sheer Force on Ice Beam. It also opens Dynamic Punch if you're feeling trolly, but its a little suboptimal.
Galvanize is just a lot stronger for an offensive ability anyway. 3 of the 5 meta Soundproof sets (Zygarde, Swamp, Scept) are immune/highly resistance to Electric anyway unless you meme Soak. Slowbro can be dealt with using Energy Ball/Seed Flare, which doubles to help remove Swamp if its a concern and the off-meta Soundproof Kyo-P (not seen that one in ages though). Cress you kinda just have to resort to teammates. Either way, there isn't enough Soundproof in the meta that Manectric could otherwise bypass with Boomburst.
For Blue Flare, Galvanize Boomburst is usually enough damage to most Steels, except Steelix, at which point its an option. Though you might opt for Earth Power anyway so you can also hit Primaldon on the switch and also damage FF-Steelix.
Jellicent: Can you show me those teams/where those teams are? You have a loooot of long team posts and it'd take a while to dig through them all, assuming I even look in the right place.
Edit: Thinking about it for 'Doom's MEVO set, it might work best on a Sun team as a "second setter". That way it can reset the weather once for its team, giving it a bit of a team support role, while also not losing out entirely on its weather/ability if it gets forced out or the weather changed. This is purely conjecture though.