The is no reason to ever do a paper certificate.

They both work the same way.

When you get the paper certificate it has a PC you enter when you make your reservation, just like the e-cert. So no you can not stack PC.

You can use your CDP and get benefits (So discount on Sirius with AAA, 4hr window on Amex etc...)

Certificates come in the name on the account.
I do not think you can give them away.