Have you tried SmartDigest Ultra? My pony had loose manure for several years before a vet suggested I try a supplement for it. SmartDigest Ultra is the one with which I’ve had the most success. He still gets the occasional loose poop, but it has been much, much better. I am not sure where you get your hay, but I know that my pony is especially sensitive to the fresher hay cuts. He also had some of the liquid gas/manure issues when the fresh grass started up this spring. Since my barn grows/cuts its own hay, the fresher cuts this spring and summer caused him to have some of the liquid gas/manure problems again. It cleared up after the grass/hay were less fresh.
My pony has been tested for various things, and nothing significant has showed up. We think that he may just have a sensitive digestive system or prior damage from worms that happened earlier in his life.
Also, for the wet manure on tail/down the legs – I clean with all-natural baby wipes and apply Balmex to keep the area from getting raw.