To exemplify the pressure issue, here's a plot from pressure measurements of how pressure changed with seating depth for one bullet shape (round nose), as it goes from touching the lands on the left, to very deep on the right, all with the same powder charge. You can see that, for this rifle cartridge (.30-06), pressure gets lower as the bullet gets away from the rifling and goes deeper into the neck, up to a point, then as you go still deeper, it rises again.
This is thought to happen because the deeper seating allows gas more time to bypass the bullet before the bullet gets to the lands, and that stalls the rise in pressure. But when it gets too deep, it takes away too much of the powder space, so it finally gets to the point that the amount of gas in too small a space overcomes the bypassing effect. The rise in pressure due to seating too deep can be corrected by reducing the amount of powder used.