86 F is the high end but if the temp is upped gradually, bettas can tolerate even 90 F for short times. Mostly, I try to keep mine at 78-80 F. 86 F is good for dealing with ich and other external parasites.
Cotton fin fungus, also called fin melt, is saprolegnia and is pretty much the only true fungus encountered in our fish. IAL will help being an antimicrobial but if you think Shigeru may have this, you could try malachite green. Another product that might help is Betta Revive. Meds in general don't work on funguses (fungi?) but the mineral treatments such as malachite green do seem to have an effect and are generally harmless even if it turns out that isn't sapro after all. Just be careful and maybe treat in an old tupperware or something because malachite green stains and you don't want to accidentally inhale it either. I think it can also set back your biological bacteria as well.
I know O Street has Kordon Malachite Green and Petco carries Betta Revive.
Did he seem to have the white trailing stuff before or after he went into epsom salt and treatment? If he had it before, then it's definitely possible it's sapro. Or, if he had a tear on his fin from biting or getting it caught on something, sapro tends to show up a lot around open wounds so that could be a cause too.
If he's doing okay digestive-wise, then he should be just fine without the metro now.
Before treating him with malachite though, I'd give him a few days in clean water to flush the metro completely out of his system.
Okay, is there anything I can do in the meantime to make sure the sap doesn't spread? because there's a huge fuzzy clump of it on his tail now. :( It's also quite close to his body, and I'm afraid it'll transfer. ;_;
Also, I plan on getting the malachite green tomorrow. Is there any helpful dosage recipe or can I follow the instructions on the bottle?