Last month I signed up for the SPG AMEX promo of 10k pts on first buy, then 15k pts after spending $15k in 6 months. Then recently I saw the much better promo of 10k initial pts plus 20k after $4500 spend in 90 days. Spoke to a variety of people at AMEX and none were able to transfer me to this promo. They put me on a 4 month review to see if they would transfer me to it (that doesn't do me much good if I still may need to be spending $15k in 6 months, which is daunting enough). Anyways, my wife is also an added card on that AMEX. Last night I figured I'd roll the dice and see if she would get approved for her own card under this better promo (not thinking it would happen since she was on my card and lower 700 credit score). To my suprise she was accepted. Which brings me to my questions:

1. We're not going to have enough spend to pay $15k toward my amex in 6 months, plus $3k to the Chase saphire in 90 days I just got, plus $4.5k towards her Amex in 90 days. So, I will just cancel my Amex and forgo the 15k bonus pts (I know, not a liked thing on this board). Would I be better off waiting to do this until later? Would Amex yank my 10k bonus if I cancel too soon? How long to wait?
2. I would like my wife to add me as an extra card holder for her AMEX to increase spend on it. Does adding extra card carriers conflict with people already having that type of card? Or is the only real result that it really increases the person's credit limit on their credit reporting?
3. Once I do cancel my AMEX I will likely have about 15k points on it (assuming AMEX doesn't yank my 10k bonus). What ways are there to keep my account and points active? Is it also an 18 month activity thing? I haven't run into an SPG dining/shopping site like the airlines have. Would it require making sure to stay at a hotel and using my acct to re-set the acct to 18 months? Or would I be best to reach for 20k points, transfer them to an airline, and cancel then?
4. I take it SPG points are not transferable to other users?

Thanks you from someone new to this game.