I don't know , but I'll tell ya, I'd like to see Druidism make a comeback. the Romans did quite a number getting rid of certain powerful faiths, and establishing new ones too.
I'm pretty sure you can. I don't see why not, but I'm neither Wiccan nor do I practice Druidry. Many pagans are trans religious where they are feel called to deities of different pantheons so I would have to say that I don't see any problem with it.
In my experience the actual practice of Druidism takes place as part of a community whereas Wiccans can be solitary or part of a coven. In either case you can participate in both unless your specific group has rules against it.
Believing in both is something else. Wiccans and Druids use different systems to describe the subjective aspect of reality. By trying to be both you would actually just be flipping back and forth. You can pursue the study of both but you should narrow your focus after awhile and develop your own path however close it comes to either.