1) I would choose Omnigrid for longevity and the markings show on dark fabrics. Most of mine are EZ for price considerations and I still have my original 6X 24 Dritz. Never too many! LOL
2) I use the 6 1/2 X 24 1/2 EZ the most followed by my Omnjgrid 12 1/2 square and my EZ 6 1/2 square
3)The extra 1/2 inch as otherwise I end up without my seam allowance! The markings are easy to read and I enjoy the diagonal centering help on the Omnigrid 12 1/2 inch square.
4)Perhaps some sort of texture on one side to help keep them from sliding while using the cutter.
5) Hopefully no more than they are already. I know you need a profit, but if the prices sky rocket then they won't sell