Does it work when you’re in a group with friends?
I doubt flex queue is disabled in parties so let’s say yes 
I would jot waste it when queuing with friends. Not sure if you play with a lot of different numbered groups, but it absolutely has a, usually negative, impact on queues.
Depends on who you queue with. Gotta love the queue times with a tank player (and also the certainty that you will get a MT).
In my experience, it does not help if your group size goes over 3. Double tank and double healer still results in 3 to 4 minutes because it is trying to find another large stack to match you against.