American Express Membership Rewards - Electronic Payment Options?

Can you use someone else’s bank account to pay AMEX bills (i.e. a spouses bank account)??

I've never tried to use an account other than my own for electronic processing, but I would assume that this is possible. They don't ask the name on the account as far as I remember; only the account number.

As long as you have the account number and that person's password you can use their funds to pay anybody's credit card you want.
Which also means you can pay into anybody's credit card as long as you have their credit card number to enter through the electronic payment system.