Yup, in your case you should. The course material is trying to make a clear distinction between 'active members' (which is an important metric) and 'total members' (which on most community platforms is of no relevance). If you cull inactive members, then your total members is the same as your inactive members.
Caveat: if you have people that join the Facebook group but never participate, they're not really active members. That said, I'm not sure you have a way of distinguishing the difference using Grytics.
Yeah, I think so, because it's so inherently tied into your product marketing strategy.