I have this exact same problem, not sure which of the three issues it is because I know nothing about cars and I just bought this one a few months ago.
The mechanic didn;t have a clue what it could be. (How is that even possible?)
I don't have a standing puddle but the carpet is soaked and I am afraid of mold especially since the warm weather isn't around anymore to dry it up. Did anyone have success with any of the methods suggested?