I'm wondering why my translation of 'It is a lot of wine' isn't accepted. It seems as though 'je' would much more strongly imply a subject of 'it' than 'there' (he is, she is, it is).
There is not a subject. It is the part of the English "There is" (meaning "there exist") phrase. There is no subject here in sight.
Yes, Czech is a pro-drop language, meaning no subject is necessary. However, the subject is implied by the verb. Thus, 'It is a lot of wine' is probably at least as clear an implication as 'There is a lot of wine'.
Nope, it is not. The meaning here is similar to "We have a lot of wine."
If you want to say "It is a lot of wine." you will say "To(hle) je hodně vína." "Vína je to hodně." or simiar.