While there are a lot of good suggestions tangential to your request, I think Kasa lights are tied to a cloud account. So, I think you could setup a cloud account for just that light. Put the app on her phone under that account. Tie just that one switch to that account. On your phone you would need to switch accounts to manage that device. I don’t know if you can tie multiple Kasa accounts to an Alexa, so you might lose that integration. Should take about 15 minutes of poking around to get it all to work, or prove it is a bad idea.

Kasa 3 way smart switches can handle this. Install the smart switch into the garage and leave this box alone. Although they are often sold as a 2 pack, you only need one smart switch on the circuit for it to work. I’ve done this several times in my house.