1) Multiple groups of 4 (WSG) or 6 (AB) means I can get in the game with a 50% or 33.3% chance to get the other group inside my battleground. How would this stop premading if I can split groups up and, in turn, gather more leaders to help lead the premade?
You're missing the point. Of course it doesn't stop premades. It stops premades being matched with pugs, which is the core of the issue. Groups of 4 will get matched in games with the opposing team with also at least a group of 4. I agree this isn't the best of 3 ideas. It is hard to implement and has room for exploitation. As for the other 2...
2) I'd rather face premade after premade instead of 40+ minutes of queue time because the only thing on the opposing team is r9 or higher while the PUG I'm in has an average of r4 or r5.
3) See #2. If there isn't enough players in a queue, you have to wait hours upon hours to get a BG pop because BGs are now level and rank restricted. I'd rather have a BG queue for people who have engineering 300/300 and the people who don't have 300/300 engineering
The goal of 2) and 3) is to mimic the rating system that was implemented later for arenas, which worked great. If such a rating based system worked in TBC, I don't see why rank brackets or averaging ranks won't work here. Yes, there were times at certain brackets where few are playing, but that's just part of the system, just like how you can't expect it to be easy to find a UBRS group in the middle of the night. In any sort of competition, there will be fewer and fewer skilled players, by definition. If I remember correctly, searches in the arena system would automatically expand to include larger and larger brackets as time went on. So no, you will not be waiting "hours upon hours".
Again, the problem is skilled/geared players are being matched with absolute noobs/undergeared. This is the core issue and needs to stop because it ruins PvP for all. I can't imagine it to be fun nor satisfying to 5-cap and win in 3 mins with zero challenge. What's the point? What's the point for Olympic athletes to go against absolute scrubs to win a gold medal?