Welcome back! I returned about six months ago after a similar break.
1. I read somewhere about there being new positions, however I see no specific information on those positions (FB, ILB, OLB, etc). Where exactly can these positions be found? Does it require certain formations to be played?
Answer - Go to the Coaching tab and select "Coaching 101". At the bottom of the page you'll see that you can switch to advanced mode. In that mode you have more control including depth chart management.
2. What would you say is a typical makeup of a GD roster. Obviously there will be some variation based on strategies, but an approximation would be very helpful!
Answer - You can use the follow as a guide and adjust over time based on your experience.
3. Would it make sense to use primarily one formation (on offense or defense), and one alone, or do you think it makes sense to use multiple formations for a team?
Note that the minimum number of players on the chart will only give you 36 players; you'll have to your extra 14 players based on your formations.
I think you'll find that you could get by with just one offense but the flexibility of being able to mix up your game plan can be helpful against human coaches. On defense you should probably practice at least two formations to effectively defend against running or passing offenses.