Okay, I reentered all the info at the "Welcome to Mail" box. I now have email. This worked without dragging Preferences or anything else to the Desktop. I've no idea why. Initially, the crash gave me the "quit unexpectedly" box; but sometime last evening it switched to the "Welcome to Mail" (sign up for Mail) menu.

New problem: while I was able to import my Mail folders (so didn't lose old emails), I don't have the more current emails that had been in my Inbox. Also, I'll have to set up the other Mail subaccounts and have no idea how to retrieve their Inboxes. Is it possible?

Almost certainly.
But I'm not sure of how.
Maybe try just dragging the messages from that folder onto the Mail.app window?
Or find an AppleScript that will import mail messages?

I don't know where the Inbox messages are. The Mail/Mailboxes folder only has the folders and emails that I kept in separate folders--the Inbox isn't there.

Thank you for all your help. Just going through this process has taught me a lot, even if I still lack for complete answers! I appreciate all the time/effort you put forth.

The Inboxes are folders that are directly under ~/Library/Mail/
and are named according to the name of your email accounts (e.g. "POP-fred@foo.org")