Thank you for providing all that information.... Poor little guy. That's a huge difference from your "before" photo on the other thread.....
So you've tried:
1) Aquarium salt - good for minor external infections.
2) API Fungus Cure - its combination of Victoria (Malachite) green and acriflavine is good for a variety of external issues.
3) API General Cure - metronidazole/praziquantel combination that's good for a wide variety of both external and internal parasitic issues. Also helps with internal anaerobic conditions.
Of those three, you only noticed a difference with the General Cure. But he then became listless, and developed external "raw patches."
Hm.... I agree with you, in that it may be worth trying an antibiotic next.
I wouldn't use the Maracyn/Maracyn 2 combination yet though. Most aquatic bacterial infections are due to gram negative bacteria, which Maracyn 2 (minocycline) treats, but it can be harsh on the liver/kidneys. Maracyn (erithromycin) treats gram positive infections, which are less frequent in aquatic environments.....
Since you have the Maracyn 2 (minocycline) on hand, go ahead and try that first. The white, raw patches may be due to a bacterial infection so the Maracyn 2 may help with that.
I'm not sure how snails do with antibiotics. You may know this already. Or, if no one comments here about this, you can ask in the "other fish" forum (or on TropicalFishkeeping.com, which is the parent website for Bettafish.com).
Also, you probably know this already but just in case --> Remove the media from your filter BEFORE adding any antibiotics. Also, I would remove the live plant(s).
(Actually, I generally just remove the sick fish, and treat it in a small hospital tank. That said, you probably already have a "method" that works for you, since you've used antibiotics before.)
So, to summarize, I'd start with just the Maracyn 2 (minocycline), and see how he does.... Keep us updated....