How long was it leaking/wet? 50/50 solution of water and bleach also is a mold killer. If it hasn't been exposed to leaking and/or has been wet for a considerable amount of time, I wouldn't worry too much about the floor. I would let everything dry out well before covering anything up.

Look up mold remediation with borax. Spray the wood with that before you button it up.

Since it is an interior closet wall, I 'd replace the drywall. It does not have to look pretty and it will be good practice for some other project that might be forced on you.

Black spots might or might not be mold. Wood flooring can turn black from water exposure only. I think I would let sleeping dogs lie in that case.

Let it dry, check for mold, close it back up.

Folks seem to have lost sight that houses are built out in the weather.

It rains and snows on and in them before they are dried in.

And a decent builder won't let it get covered up until the moisture content is at a resonable low. Cover it all up too quickly, which is all too common these days, and you get a lot of headaches!

The sealant you fixed the leak with will eventually fail and leak again unless you check it often enough to be able to replace it before that happens.