your betta's symptoms (other than the white stringy poop when he first came home) do not lead me to believe a parasitic infection is the main case here. the white fuzzies on him sound bacterial in nature.
i personally would give him a couple days with 100% water changes daily, and no medication, then re-start the betta revive or go with a different med that will cover bacterial infections.
bettas are just like other tropical fish, and other than not using certain meds (i personally will not use melafix on a betta), they can be treated just like any other fish.
the white strings coming off the decor in his tank sound like protein strands. are you feeding a lot?
stop dosing the salt. freshwater fish don't need it, and there is no reason to use it unless you are treating for ich with the heat + salt method. none of my fish have seen salt in their tanks in at least a few years, and are very healthy. on that note, using salt for more than 10 days will not cause kidney issues with your betta. not sure where you are getting your information, but it is incorrect. trust this forum before all other sources, IMO.
turn down his heater. if this is a bacterial infection, you want his tank on the cool side (74 to 76), because bacteria thrive in warmer temperatures. the cooler tank will keep that bad bacteria from being too 'happy'. bettas are fine at temps around 76 or so. my betta in my 20 gallon with other tropical fish is at 75/76 constantly and he's a happy lil guy. 80 is too warm, IMO.
so to recap - daily 100% water changes, give him a couple days break from meds, then restart with the revive or another anti-bacterial med. i've used API's 'General Cure' successfully on a very sick (near death, floating sideways) large cichlid that had a bad bacterial infection, and it worked very well. this med works for bacterial infections, and also contains Prazi, so it is effective against parasitic infections too, just in case your betta does have some nasty parasites partying it up inside him.