Hi Moose, and welcome to the madhouse..! To try and answer your questions.
Crossover, which some people refuse to believe exists, is when the scope rails, mounts or scope make a horizontal angle with the bore. This means the scope is pointing slightly to the left or right of the bore. I you zero at 35 yards, for example, shooting at 15 yards may shift the point of impact, POI, to the left. If you shoot at 45 yards, the POI shifts to the right, hence the term "crossover." The end result is that the rifle will only shoot true at 35 yards.
Cant is when you lean the rifle over to one side. Because the scope is mounted over the bore, which is inclined slightly upwards, the pellet will be thrown off to one side.
Holdover is when you have to raise the crosshairs over the centre of the target to hit at longer ranges. Holdunder is the opposite.
Parallax is when the image of the target and the crosshairs do not coincide. This happens because the scope will be focussed, in theory, at one fixed range, at distances further away, the target image may be in front of the crosshairs, and vice versa. If you move your head from side to side, the crosshairs appear to move across the target, causing an error.

It may be possible to buy mounts with a 30mm bottom half, and a 25mm top. Try phoning our sponsor, JSR, or Blackpool Air Rifles.

Gus