Hi, Moobs, I don't know that it's a waste to go AU for now. FICO 08 appears to be on indefinite hold. I'm AU on my husband's card for the age, and I've decided not to convert to joint for the time being. Once FICO 08 kicks in, if it really hurts my scores, I'll decide then.
There are some real landmines out there for joint status, especially if it's not for a married or other couple who live together and do their money management together. Even when they do, we get too many posts where DH or DW ran up the card, hid it from the partner, maybe ran off into the sunset with a new love interest.
For either joint or AU status, if one person is trying to time payments and control balance reported for scoring purposes, it can get pretty exasperating to get the other person to understand and play the game.
With joint status, if anything negative at all happens to that card, the other person is equally responsible, and the CCC will definitely come after the "innocent" party to recover its money. This includes a trusted friend getting access to the account, pulling out cash, and on and on. An AU can just bail out in this situation (except for a spouse in community property states, where they'll come after you anyway.)
That's a start anyway. You're putting your mother's credit at risk, and you would also put your own at risk if something strange happened on her end. hth
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007