Most projectors will have a slight downward throw when ceiling mounted and/or have some vertical lens shift to allow the projected image to clear the support beam. The projector ceiling mount typically drops the projector ~6in from the ceiling, and the lens centre is typically ~2in from the base of the projector. However, it really helps to have very precise measurements of intended screen location, wall, ceiling and screen height to make sure the installation will work as intended.
1) Many USTs have a limited focus range which will determine maximum image size. Optoma specs state 120in max for the P2.
2) I can't answer that.
3) USTs typically have no optical zoom.
Here's a projector image size calculator with a BenQ HT5550 used as a example:

BenQ HT5550 Projection Calculator - Throw Distance and Screen Size
Find screen size and throw distance for the BenQ HT5550 projector.
www.projectorcentral.com
Let's say that your ceiling beam drops down 6in, and is 8 ft from the screen wall. If you want the top edge of the screen to start 6in from the ceiling, then you could place the HT5550 anywhere in the room and have no problems with projected image clearing the beam. A higher top of screen might require the projector ceiling mount to be placed further from the ceiling to have the top edge of the image clear the beam.
Variable masking sounds great in theory, but there are some movies that use a variable aspect ratio within the same film...
A painted screen wall can work very well with long throw projectors.