I would say you can get it to about 2-3 feet. The issue is that you need access to the locking pulley - it works like a shade/curtain pull. It unlocks when you pull straight down and locks when you hold it away at an angle. The closer the canoe gets to that pulley/locking mechanism the hard it is to lock/unlock.
Can be adapted to some slope by different rope lengths. Best with flat ceiling.
In order to mount the ceiling brackets at different ceiling heights, you would need to measure the difference in ceiling heights and ensure the rope connecting the two points does not come in contact with the ceiling's edge. Any friction between the rope and edge of the ceiling will cause the rope to fray as well as make the lift more difficult to operate.
No, you need both screws to properly mount this. If your studs are at right angles to how you want the brackets and hoist system to face, you would need to do a different work-around -- you would need to mount a board to your joists (a 2 x 4) and then mount the hoist system to that board as the normal installation for this system is to one joist not two. If you need more information to set it up going across two beams/joists, please contact us directly, we are happy to help.