Yep, I'm pretty sure you should wait a few days after finishing an ES treatment before beginning an AQ salt one. I think that's just to get it out of their system; the two are definitely not compatible (I wish though).
Bettafix is an extremely controversial product. Some people say that it's incredible, others say that their betta died from it. I heard someone here on the forum say it's great for finrot if you've got sufficient surface agitation, because if you don't, the tea tree oil builds up on the surface and gets into their labrynth organ when they go up to breathe. I'm not sure if Bettafix would be helpful for healing inflamed gills or not...I think melaleuca is good for external healing in fish so maybe.
Bettafix/melafix/primafix products are DEFINITELY dangerous when overdosed, so if it were me, I'd (a): make sure that the filter current agitates the ENTIRE SURFACE of the water, and (b): underdose, maybe even halving the suggested amount.
Since his bloating has gotten better so rapidly, I'm pretty sure that my guess of early dropsy was wrong. I'm thinking now that it's probably some form of ammonia poisoning, or maybe a bacterial infection. The best thing you can do right now is probably large daily water changes, to keep all of the parameters perfectly in check. The ammonia neutralizer might help too.
I hope this is useful. Keep in mind that most of the stuff I'm telling you is based off of research, not experience. I wish someone who has been keeping fish for longer would come on here and lend a hand, but until then, I guess it's just me.