With Seiza my instructor suggested putting a pair of wool socks under my ankles to help support the pressure. Also I find that it is easier for me to sit in Seiza after a workout or warm up. Try also to concentrate on relaxing past the pain starting from your sholders and ending at your hara. This takes some of the pain away at first, and more of it as time and experience passes.
With rolls think like the spinning figure skaters. As they spin their heads don't. If you watch closely they turn their heads ahead of their spin, stop their head and focus on something. When their body spins around to the point that the head has to move they turn their head ahead of the spinning body again and repeat this through out the spin. Try the same with the roll. focus on a point ahead of you till the point of the roll takes place. then tuck your head and roll. When you come out of the roll immidiatly train your focus on the spot you were just before the roll. Do this slow at first because one can focuse too long ahead of you before the roll and bam you end up landing on your head. For me the act of concentrating on where I'm going to end up as opposed to the roll itself is what makes the difference in whether I get dizzy or not. Hope this helps