I'm not for certain, but I believe the answers are as follows:
1) Yes. Book them together on the same reservations. Hopefully this can be accomplished online, but you might have to call.
2) No. Unless you don't plan to use the full value (in which case using the amex card would be best as there would still be $$ on it), the best option would be the delta e-gift card. This does not operate like the vouchers. It's treated as a payment method. I've selected the $200 e-gift card as a DM before and combined it with a voucher.
3) Depends on when the travel is. If it's post May 16, it would all be priced into the C+ fare. In that case, yes. If it's before May 16 the e-gift card would work for sure, but I'm not so sure about the vouchers.
4) I believe so, yes. EDIT: Maybe not, per others
You could probably figure out most of this for sure by doing a test booking on DL.com.