Temporary Child Visitor Discount added to card – Senior equivalent?

We don’t live in London, but we have three elderly unregistered normal Oyster cards that we use between our family and friends when any of us visit London. We just top up these cards with credit as and when we need to.

Usually, it’s my son (15 yrs old) and me, and we ask a staff member at an underground station to add a Child Discount to one of the cards for a few days to cover the duration of the visit so that he can benefit from child fares. My understanding is that after a few days the child discount “drops off” the card and it reverts to being a normal card.

Next trip is this weekend and I’m bringing my mother (80+) with me. My question is about whether there exists the possibility of doing exactly the same as I do for my son, but for my Mum so that she benefits from the oldie discount. I know there are ways of linking a senior railcard, but I don’t want it as long term as that; I just want the oldie discount to be added for the duration of the visit, and for the card to then revert to a “normal” undiscounted card so that others can use the card on future visits to London.

And slightly off topic, am I right to think that my Mum can use her free bus pass (issued outside London) on London buses? (subject to the usual post 9:30, weekends, etc)

Many thanks for any advice.