I'm not an expert on chytrid and have never seen/treated it personally, but from everything I've read it shouldn't be able to thrive above about 73. Since pacs shouldn't ever get below 75 it shouldn't be able to live in a pac enclosure. Given the temps and the fact that almost all pacs are captive born and bred, I just think bacterial infection is far more likely than chytrid.

So, with the water pouch are you talking about just like an area under the skin that looks like it's holding water, or something that's very wrong looking? If it comes and goes maybe just leave your camera beside the enclosure so you can snap a pic next time you see it. Pics are really helpful for evaluating stuff like this. Pacs can retain a lot of water at times just as a normal part of living. Without a pic I feel like that may be what you're seeing.

If the new sore on his back looks like the one in the picture you posted before, it'd be fine to put painkiller free neosporin on it.