Removal of 1/2 of each tile is to help the distribution. If they were all left in there, and you take less out, there is a higher chance that some wont be see at all.
Not sure about the 40 / 48
I'm not sure. You randomly remove the same amount of tiles from each family so the distribution between the families stays the same.
The points on the tiles are 2x5, 2x4, 3x3, 2x2 so there is a slightly bigger chance to remove the 3 point tiles but I think this has a really small impact.