Hey Old Mechanic! Thanks for looking at this!
By George, I think I've got it!
I ordered a new clip, thinking "All of this work over a two-dollar part!"
The clip was still there and I took everything inside last night, trying to puzzle it out when we had a bit of an emergency last night. Everything is fine now, but that was an unavoidable distraction. When I put the pieces together, they were steadfast and immovable, until I tried assembling them in my car, then the lock cylinder kept falling out!
However...
Removing the clip and inserting it last seems to work! I can lock my door [normally] for the first time in months!
Thanks so much! Have a great day!