I don't have my mom in my garden much!
I use three kinds of labels, plastic tags cut from yogurt cartons, plastic and wire flags, and little paper price tags that have a string and paper that can be tied on.
I regret the plastic it leads to microplastic in my garden. I use a very permanent garden marker on the plastic.
Wooden tags and wooden stakes could replace the plastic. It might be nice to dremel or carve in the name rather than use a permanent marker with who knows what chemicals attached. That might be what I go to to eliminate the microplastic issue.
Seed saving is cultural. Largely what you are describing with your mom is a cultural shift you are trying to implement. My parents used to save seed by putting unused portions of seed packets in a jar and planting from them again the next year. Very little seed was just saved plant to seed in their generation. Your mom is in that same mind set where an overripe vegetable is starting to go to waste. Unfortunately I sometimes over save seed! I think sometimes I could supply the entire local seed library.
The Hidatsa culture as described in the book "Buffalo Bird Woman's Garden" had a leaf folding technique to mark seed squashes. This was understood by all the gardeners- that squash is for seed, ok, moving on. So basically what you are asking is how to get your mom on the same page as you in regards to seed saving. A large wooden stake that says in big letters "Mom Don't pick this one it is for seed!!!"?? Generally though just talking to your mom quite a bit and accepting that there will be mistakes and some conflict when two people share and collaborate on a garden. Still remember a squash that my mom and sister ate as a summer squash that I wanted for seed back about 2004 or 2005.