I wouldn't think so, usually once the symptom of drospy (edema) shows up to the point of scales sticking up it is usually fatal with 5-7 days more due to system failure and kidney shutting down. So if he has survived for 3 weeks I would say it is bacterial in nature-look for an antibiotic that covers/treats both gram negative and gram positive bacteria. The Epsom salt can be helpful for the symptom of dropsy (edema).
I use Epsom salt 1tsp/gal up to 3tsp/gal (not aquarium salt) for no longer than 10 days along with daily 100% water changes
I keep them in the Epsom salt 24/7 a treatment not a bath/dip
I pre-mix my Epsom salt 1tsp/gal in a clean 1g jug to make dosage and water changes easier.
My betta is still living although very bloated. The maracyn two and aquarium salt do not help with the swelling. He floats at the top and and sometimes eats. Is there anything else for the bloating?
Awesome, the betta food should not be a problem for the Endlers
Lucky Endler fry too, my Bettas gobble up guppy fry pretty fast in my tanks...lol....but Bettas have never been a problem for adult guppies...I have lots of both species all in the same tanks and only guppy newborn to 1 week old fry are in danger....lol....once a week or so old they are too big and fast for the Bettas to catch...my natural population control...laffs.....
My RCS adults have been safe with the Bettas but I am sure they do get some of the shrimpletts....except in one of my experimental 20g spawning tanks....adult shrimp don't last long with 5 females and 1 male even with all the plants, but in the 75g with all the Bettas and guppies the shrimp thrive...so tank size has made a difference in my case and survival of Shrimp.....
Through his gills, but floating on the back is not a good sign sadly.....sometimes the injury can be worse than you think, you may want to lower the water level some and put the fish in a dark quiet place and hope for the best....but honestly it doesn't sound good.....