What's the minimum number of squadmates and missions I need to be able to use the Omega 4 Relay?
You need at 2 for your team and 1 for each role they play during the rescue/assault that are not in your party. As I recall you need someone to do the infiltration, another to do the biotic shield and one other for the fire team, so that is at least 4 5 allies. I don't recall offhand any other roles.