Set up twin households/rooms/mutliple adult users of a household, so elderly grandmother can use alexa without hijacking my phone?

I have prime. I bought an echo dot for my elderly grandmother under my name. She is blind and has dementia. We live in the same building but basically upstairs and downstairs flats, close enough to be on the same wifi. I can run downstairs to help her anytime but I want to set alexa up so that she can be relatively more independent. I just bought her a Fire TV Cube so that she can turn on the TV through voice command. I want to have two different ecosystems of music/tv/etc playing for upstairs and downstairs.

When I try to make her echo do something (like play from spotify), it basically hijacks my spotify account on my phone. Even if the command came from her phone. I already have a spotify family subscription with multiple profiles. My mother has a bunch of Audible books from 10 years of membership we want to let her listen to. Currently, I and my mother are the two adults on the Amazon Household. I'd like to open my grandmother's echo, make her an account, and log into my mom's audible account on that echo.

Can we set up my grandmother as a "teen" on the household with her cell phone, so that she can listen to our shared content and play her own music but not buy anything on prime? I can just buy stuff on her behalf.

Or should we dis-associate her Echo and Fire TV cube from my household altogether and start over with a brand new household through her cell phone? If disconnected households can still do the drop in feature that would be nice but not a requirement.

Any help would be appreciated, thanks.