no worries with all the q's.
I have not re-fitted mine yet due to the weather.
The car still locks and works fine without the power sounder attached to the car.
I believe if your car gets broken into then the horn and hazards still flash (i am not 100% sure about this)
I had to re-solder the new batteries on the board which was a bit fiddly but with a steady hand and a small tipped soldering iron you should be fine.
I then re-sealed the unit with some gaffer tape.
If you want any more advice just give me a shout.