I have two questions pertaining to purchasing AA tickets.

1) As a means of liquidating some Visa GC's that I plan to buy, I'd like to use them to buy flights. An AAgent told me that you can use multiple forms of payment at the airport, but not online or over the phone. How does this comport with what people know? Do they have to be physical cards or will eCards do?
2) I got a voucher for denied boarding. The document says it can be redeemed by phone or aa.com. Someone in the boarding area said they were transferrable but only if the owner and transferree signed them together in person. How does that work? It's possible that my voucher is different than theirs because mine was voluntary and theirs wasn't.