Cat's correct. The in/on walls need gravity to 'lock' them into position. Upside down mount and you risk them falling to the floor. eek
I wouldn't worry too much about the tweeter height difference. As close to the same height is the best case scenario, but most systems out there have this type of difference and sound very well. Just do the best you can within the limitations of your room and enjoy!

Dan
On-Wall M5HP LCR, QS8 & EP500 in 7.1