I often connect from MAN to a longhaul in FRA with the connecting time of 1 hr 35 mins and have never had any problem (except on Christmas day but that's another story). Even if your BHX flight arrives 30 mins late you should be ok.
Although it's a long time since I connected from a flight ex-UK to a longhaul departing from an A gate, I think the route to follow will be as follows: From the bus arrival gate in B you go up the escalator or the lift to level 3, follow the signs to A51-65 gates (the same direction as C gates) which should take you to the Skyline train platform, take the train to Pier A and walk to your departure gate. There may be security check as you get off the Skyline train in Pier A.
Although you are connecting from LH to UA, you may like to take a look at the following thread in the M&M forum.
http://www.flyertalk.com/forum/miles...on-thread.html
I'm afraid I cannot answer your other questions.