1) Depends on which class you're playing with, most of the time the DPS on your team will know which target he can kill more easily
2) Pretty much, it can also be used to avoid cc like polymorph, fear and clone and can also be used to land a fear (feather, guise, fear)
3) The easiest thing to do is make sure you always have SW:P on the target and also use Holy Fire on CD as this gives you stacks of Evanglism for big heals when needed.
4) you really need to use comms.
5) Happens alot in group finder, just find 1 guy that you enjoy playing with and add him on Btag, this is how I found my teammates
6) It's great to play alot of classes so you know how they work (EG a rogues Red buff, feral CD's etc.) but you really should main 1 class so that you get better at the one class.
1750 in 2's and 1550 in 3's should be an easily obtainable goal for yourself, before this season I was 1550 max but from actually trying I managed to get 2.2k (which I know still isn't great compared to most people on this forum but hey for me it is good!)
Just keep practicing and I'm sure you'll easily get what you want