How do I switch from one account to another so that the accounts are totally independent?
Now I see what you mean. You can only have one Thunderbird at a time within which you can have multiple accounts. But if those accounts need to be inaccessible between/among users, you're SOL.
If one of your other solutions allows such, you're better off going with same.
If your spouse remains refractory in the ways you've described, I'm surprised that she wants to put
you through the wringer rather just allow "side by side" email accounts within the context of Thunderbird.
C'est la vie, je suppose.
