I can only answer the seed phrase question: it can be finicky, but both wallets are supposed to work from the same key. If it goes wrong, most issues can be fixed by reinstalling from the same seed again. Issues can come from bad wallet code, a bad node, a bad internet connection, a long time (months/years) leaving the wallet closed, or any combo of these
Discussion
Hmm - and I know I could just create a separate wallet for her with a token amount of sats so she could get accustomed to using it...just seemed like it was theoretically possible (but perhaps not practical) to have a shared wallet. I definitely do not need to create a bad user experience where something doesn't work and she panics as I restore from the seed phrase again!
You can also have any number of extra wallets just for viewing and even if they ever have trouble syncing with the network, they'll never interfere with the transacting wallet