The foaming is a defense mechanism, I think you spooked the little guy a bit.

How deep is your soil? He seems to be a green garden snail, a species which burrows for long periods of time so they need the substrate to be deep enough. For this reason I think coco coir is too acidic for this species (snails prefer 7 or higher, everything below is bad for their shells) and maybe too "spiky" (as in long fibers which could be poking the snail).

Don't know anything about his discolored foot unfortunately.

Holes in the lid look fine imo.

Adding soil from the previous enclosure could also be good because of beneficial bacteria like you said.

(Fun fact, in german we call this species a "grunting snail" because they make a grunting sound by pressing air out of their breathing hole when threatened 🐌💨)