I won't argue the advice you got here, though I do find it interesting, because the general consensus I've always seen with members and on the site's FAQ say 4-6 (or more in some cases, but not less) per 10g. But if it doesn't seem to be causing you big issues since the tank is large, glad its working out.
It would stand to reason that if something is being fed plenty, it wouldn't lose weight.
But I'm sure you'll have time to fatten her up while she recovers.
I'm not sure what you mean by the current. Current alone shouldn't do too bad a number on a female's fins if they are reasonably short, which hers seem to be. However, if you mean that she's been getting her fins snagged on the input of the filter, there are a few ways to correct that, protecting her and the other fish. If you use filter sponge or pantyhose, you can cover the outside of the output, permitting water to flow through, but saving fish from getting fins trapped. I warn that it does reduce the current (not so much the foam, but the pantyhose) sometimes, and the biggest complaint I see with it is that it decreases the amount of debris pulled directly into the filter, so you need to gravel vac. more frequently. It may not be ideal for your tank, but it is a thought if it becomes an issue for her.
Before you put her back in the 20g, especially if there is even the slightest concern about nipping between the females, you'll want to pull the other two girls and re-arrange the decour a bit. When a female betta is removed from an established group for any length of time, the group will see her as a newcomer, and will bully her (or worse). By re-arranging the decour, you confuse everyone into thinking they're in a new territory, so while there may be some agression, it won't be the severe territorial agression you'd see if you left the two females in. After all, it'd be a pain in the butt to get her fins all healed, maybe do something about the filter, then put her back and have her old "roommates" beat her up.
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