I used #12 but you could go to a 1/4 if you made the hole bigger. there are 3 sets of two holes you could use 6 screws, I used 3. The material and thickness must be considered too. Sorry, I need more information to suggest an exact screw.
No, I would recommend #10 x 3/4" Philips FLAT HEAD wood screws
Yes, the leg and mounting plate are a set. It is roughly 5 inch diameter but somewhat triangular shaped with 3 sets of two holes. they are 3-1/2 to 4 inches apart depending on using the close or far set of holes.
with no additional height adjustment on a hardwood floor they measure 27 5/8" from floor to underside of desk/table. You can then add additional length by unscrewing the large feet
Yes, they actually screw into the plates so easily removed
I suggest #10 x 3/4" Flat head Philips wood screws also from HomeDepot
The "height" adjustment is via the screw levelers at the bottom of each leg.
28-1/2 (- 1 inch) 27-1/2 to 28-1/2
It was about 4 to 5 inches in a semi-triangle shape