Skeledirge DOES work for some Tera Raid Battles; I have seen it put to good use, far more so than I've seen Meowscarada and Quaquaval (probably because Skeledirge has better bulk). Torch Song combines with Throat Spray to become an accelerating mass of damage that can overwhelm even some 5- and 6-star raid Pokémon, though it is vulnerable to when the raid Pokémon resets their stat boosts, especially since they won't be getting that Throat Spray back until the raid ends. Unaware ignores all defense boosts on the opponent when attacking and all attack boosts when getting attacked. It does apply to stat drops as well, but your Sp. Attack boosts using Torch Song will still apply.
You are only allowed to Terastallize once you have landed 3 or more attacks. Abomasnow may get knocked out by then, or be so close to getting knocked out that Terastallization becomes impractical. That being said, I could see Abomasnow working against some physical hitters, especially if you can find a way to heal in the meantime and it happens to be a Pokémon whose natural types Abomasnow happens to be strong against, like Ground or Water.
I have seen Bellibolt, and it works great in Tera Raid Battles, albeit I don't see Bellibolt that often and it is pretty reliant on Electric-type attacks. Since it doesn't come up that often, I can't really say how well Bellibolt works, other than I more often see Bellibolt falter due to poor usage rather than it being an unsuitable Pokémon for raids.
All in all though, there is no substitute for experience. Rather than sit and ponder what to do for a raid, you should raise something, no matter how crazy the idea, and go out there and try some yourself. These things are complex and varied, and no one Pokémon can be a good answer to them all. I myself have over two dozen Pokémon I use for Tera Raid Battles, and that number is still increasing. What works wonderfully for one will work poorly for another. Once you clear at least one raid of 5 stars or higher, it will get easier, and you'll understand them much better.