1) Some of the demos don't work in v1.12a/b. It states which ones aren't in the demo video menus.
2) You need to turn on the electrics before starting the engines. I think it's Shift-L? Check your key setup in the options. Also, make sure your throttle is at idle before starting the engines! Otherwise you'll be going through the startup procedure again, only after the engines have stopped spinning - and that takes time!
3) Erm, this might just be cause when the aircraft is parked. Probably won't happen when using the flaps after the engine is started. Never had the problem of tipping over when putting the flaps down : P
4) There was a recent thread on this, which includes a fix I think. It's on this forum somewhere.
Boberro: perhaps your problem might be forgetting to start both engines? That would cause a yaw problem and cause the HUD to show a turn to the left or right? Not sure, just throwing a suggestion in there!