Sorry to hear of your troubles.
As already mentioned your Brera will be a 200bhp model if it's an 06 or 56 (no matter what the dealers might tell you!). The 210bhp Breras have a sticker on the drivers door pillar stating they are 210 if you want to check
1. the rear knocking - silly question but try grabbing the nearside exhaust tail pipes and giving them a good shake from left to right. Sometimes a poorly aligned rear box or a worn mount can cause the issue you mentioned as there isn't a huge amount of pace under there. If it knocks then thats more than likely the issue.
2. sounds like it could be a battery issue, or maybe even the control module (if they have replaced the heater coil that is).
3. is there any chance someone might have remapped it before you got your hands on it?
4. sounds suspiciously like the dreaded inlet manifold issue
hopefully not, but it does sound that way. Have you pulled the air filter off and checked the arm on top of the inlet manifold is still connected properly?
The inlet manifold problem doesn't always flag a fault on the dash and would meet all the problems you have described
Lastly, save yourself the hassle and get the car to Ned at autolusso
(he isn't too far from you), he will be able to sort it in no time and would save you time and money to be honest.