I appreciate all the advice. I read through this entire thread again and mucked around.
I started by opening my oldest known good MNY file without issue. I then went through each of my different copies of MNY files which I had by going to File→Open. In between each file, I made my way to "View Downloaded Balances" so that I could write down what the latest statement date was.
After going through several files successfully, I finally made it to my newest/most current MNY file (to which I had recently assigned a password) and I was able to enter the password and open the file.
Yay!
I then closed Money and opened it again, and it asked me for the password. I typed it in and it told me I had the wrong password (but I know I typed it in correctly) and would not open the file!
I tried opening the old known good MNY file again and going through the process again, but eventually I got to a MNY file for which it asked me a password (even though that file didn't have a password) and couldn't get back to my current MNY file successfully.
I then realized that in some of my attempts, I double-clicked on the MNY file, instead of opening Money 2000 and going to File→Open, so I closed Money and tried that - and lo and behold, it worked!
I have since tried several times and confirmed that I can only open my MNY file by double-clicking from File Explorer, but not by opening from within Money 2000 itself!
With respect to the version incompatibility:
I installed Money 2001 and Money 2004. Neither of them would import my Canadian Money 2000 file. Both reported the message "Money cannot open this file because it was created by an incompatible version". To confirm that it wasn't an issue of a corrupt file, I created a new Money 2000 MNY file with a few new accounts in it and then tried to migrate that file to Money 2001 and 2004 and it still wouldn't open.
So I'm convinced this is an incompatibility between the Canadian and US versions of Money, and maybe specifically the Money 2000 Deluxe Canadian version, (I think that was the last Canadian-specific Deluxe version released - which is why I have clinged to it all these years).
I realize that it is possible I am "living on borrowed time" with Money 2000, and I may eventually need to export QIF files for all my accounts and import them into Sunset, but I'm stubborn and I'm going to keep making backups as I go and keep my fingers crossed.
Thanks,
Jeff