I have discussed my suspicions of this being an "inside job" with the other family members, and all but one independently responded with "wow, that was my first suspicion, too!". Apparently I was the last to reach this conclusion. And still don't want to believe it.
If true, it wasn't done for money but for revenge. This is someone with whom I've had a great relationship for 40 years with not so much as a single cross word spoken in all that time. There must have been some psychological break, perhaps starting with my wife's passing. That trauma changed all of us.
Back to good news. I now have a proper steel door and deadbolt on the garage/office/studio, and that's a relief. Not only that, after my stepson installed it for me he wouldn't take money for either the hardware or his time. Not only that, I think I may have talked him out of voting for Trump. Things are looking up!