1) What does optimally mean? Do you mean write to the card as fast as possible? Then yes, because of course UDMA7 is going to be a lot faster than a cheap CF card. But the other fast (but not fastest) cards will still work great.
2) Depends on what you want, and if you need high speed writing. If so, you'll need to have a card that can keep up pretty close to the CF card.
3 & 4) I've never shot with a dual card, so not sure what I'm going to do, haven't thought about it much yet.
5) Ah...what are your needs? Do you need to flush out the burst mode buffer as fast as possible? Or do you take a couple of shots a second at most? If the former, it might make sense, if the later, any of the reasonably fast cards should more than suite your needs.