1) The tweeter heights do not have to be 100% equivalent in height.

2) Yes, angling your centre speaker downward a bit will help in regards to your first question.

3) You really have to move the sub around the room to find the best sounding spot for your situation. However the most common locations is placing the sub right at the front beside their main speakers or right at the back behind/alongside their listening area.

4) That cable should be fine.

Enjoy your new system!

"Those who preach the myths of audio are ignorant of truth."