Unable to open Mail in macOS Catalina
When I was 99% drafting the question above, I came upon a solution adapted from this post on AppleToolBox.

Here are the steps:


Go to ~/Library/Containers/com.apple.mail/Data/Library/Saved Application State and delete com.apple.mail.savedState.
Go to ~/Library/Containers and drag com.apple.mail and com.apple.MailServiceAgent to your desktop.
Go to ~/Library/Mail/ and drag everything in that folder to your desktop.
Restart.
If it's now working, trash everything on your desktop.


Because I had already tried deleting accounts before finding this solution, I had to re-add all of them to the user, but it otherwise seems to be working normally again.