This is what we do, we have 3 senior developers, 2 intermediate level developers and 3 juniors on our team. Even before we hired the juniors we ALWAYS had GoToMeeting up and conversed throughout the day and worked together on projects, it's more imperative with the Jr's so we'll usually pair someone up 1:1 with them on a separate call to mentor them on a project. We haven't felt a need for a physical office except as an occasional meeting space (which is why we are working on getting one built-out in the city where the majority of our remote team lives).

Yes. We have found it is very frequently no where near as successful as in-person mentoring.