For question 1-download SH III Commander and if you are using GWX 3 download the Commander file for it also.
Question 2-rarely you will get a wounded guy and the medic helps with recovery time. It is rare for wounded, in my case, so I use him as my gun captain and sometimes watch duties.
Question 3-as far as I can tell-it is nice eye candy. I have not found a function for it as yet, except maybe it helps with map contacts if you play that way.
Joke-
I am sure others can fill in the details if I have missed anything.
Good hunting.