2: i wanted to try building blacephalon + mega heracross today, but i ended up trying some other stuff because it was pretty tough. even though av ttar is gonna decline in usage due to naganadel being banned, i think it's still something solid to have on this team in the post-ban meta. without it, this team would probably be a little too reliant on ferrothorn defensively handling things that can annoy specs blacephalon like tapu koko, greninja, ash greninja, rainshit, etc.. even then, it looks like cores combining those (or cores that have something that can lure and/or exploit av ttar or ferrothorn) will be a pain. anyways, this is a solid build since that can usually be handled with good playing.
3: obviously this is the same framework as the above team, but i like it somewhat better because mega lopunny adds some much needed speed. while blacephalon isn't slow by any means, having a not-so-speedy breaker + a slow breaker (hera) without sufficient speed or defensive backbone to make up for it is not great. lopunny helps to make that not an issue while still filling the same basic role heracross fills (being a fighting type lol). additionally, it can be a decent check to greninjas and tapu kokos (non scarf / water shur ofc), indirectly taking some pressure off of your defensive backbone. cool team.
5: slowish (and choice-locked) breakers + voltturn is definitely not a new concept, but it's great because it works. this team also does the straightforward idea of blacephalon + fighting type (lopunny again), but you also have a pretty standard bulky offense framework to go with it. obviously when you use bulky offense teams with that av magearna + defensive landrous-t core you need to be wary of that getting overloaded. however, i don't think that's much of an issue with offensive pressure maintained. still, i feel like you really need to keep the offensive pressure up against tapu koko and greninja. mega lopunny and scarf greninja seem to cover this on paper, so i think you're fine.
i really like the idea of kartana as a partner for blacephalon since it provides a secondary breaker and defog, as you highlighted in your post. seems really cool on paper.
6: other than fighting types and steels that can come in on choice-locked tyranitar and gain momentum (by setting up, attacking, or just switching out w/ a predict), the most common partners i've seen for blacephalon so far have definitely been mega latios and zmove heatran due to how they can pressure tyranitar (and opposing heatran in latios's case ofc). while the surprise factor of mega latios is pretty much gone now, it's still good at threatening those. due to the fairly standard bulky offense framework of this team, i don't think it'll have any problems if played competently (but it's finch so that's highly unlikely :3). choice scarf blacephalon is a really cool choice since it can easily clean with its spammable stabs ofc. i also really like how this team has spikes support. really solid looking team.
7: a friend and i tried building around various torn-ts + blacephalon, but we gave up. fightz seems really cool for luring tyranitar on paper since without the noticeable life orb recoil, people might think you're av (which usually doesn't run superpower), but at the same time, i don't really think many good players are going to be that risky with their tyranitar vs. a blacephalon team. i still think this is a fine option, but it just needs to be played really well (not trigger happy for sure). i'm inclined to agree with your other option in the description because, as pointed out above, this general framework does seem pretty risky vs. zygarde builds. if you did that change, you could probably also just run rocks mega tyranitar + spikes ferrothorn now that naganadel is gone. while you are pretty defog reliant with blacephalon (and idk how reliable of a defogger landorus-t is yet), even getting a little big of spike damage on some stuff could be nice since you can fit rocks elsewhere.
9: super wild team with a lot of cool techs and ideas for the nagandel meta. sadly, i don't think heatproof bronzong will be as useful now that naga is banned. still, this team looks super fun, and it doesn't seem to have any of those weaknesses unorthodox sets/mons might bring. substitute blacephalon is super interesting, and i really want to see how that'd work in practice. cb zygarde seems like a really cool partner since tangrowth usage is lower, and blacephalon handles that and mew anyways. it's also really good at pressuring tyranitar, heatran, and pex, which is nice ofc lol. really fun looking team.
with the naganadel ban in mind (since it seems that the project is taking that into account), i'm gonna vote for team 6.