nitrite of 5! Do lots of pwc's. Nitrites at 0.25 is dangerous, so you need something like a 99% pwc just to get it down to some half decent level. <At least 2-3 70% pwc's if you are keeping the fish in the tank, or remove fish & change all the water!
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That nitrite has to come through ammonia, so there was prob an ammonia spike before you caught the nitrite spike. That was prob the cause of the red gills. Major overfeeding might have caused the "mini" cycle, or may be something had died & decomposed. <Mini in time frame, not in spike levels in your case.> I would do a major gravel vac to get rid of any possible stuff hiding in the gravel.
Snails should be OK with the low level of salt needed, as long as the salt is well dissolved & you don't get grains on the snails. I wouldn't go much higher tho. <Treating nitrite, you only need something like 4-8 x the nitrite level, so even at 5 ppm, that is only ~50 ppm of salt = 0.005%, so you can go even lower than the recommended 0.05%.>
With goldfish, I drop all the way to 68-70 to increase oxygenation. You prob can't go that low with guppies! May be 76-78. I would only drop 1-2 degrees a day, or the fish may be shocked. But it is good that the sickest fish is no longer gasping. If you just keep the water quality pristine with pwc's, she might just turn around.
