The first paragraph sounds like you are describing using Firefox sync or chrome with a Google login.
It's not your threat model but it's someone's threat model and a chromebook does that better than anyone else, and your mom can do it.
Now you can even use an alternate Android or Linux browser instead of Chrome.
You can have a second account that is empty that you log into and isn't connected to anything.
My mom can still do both of those.
Doesn't matter, you've protected your clients, that's what matters.
The worst case scenario is they seize it and prevent entry, which can happen with a blank or non-blank machine. Better it happens with a blank one.
> They would probably make you log in and sync before they let you enter the country.
This would be where you just don't enter the country. In the other scenario, you don't get in and they get to keep the machine anyway.
It's a great OS too to give your parents who only need it for web browsing. Less maintenance and tech support.