As I said the defence+ under the control of cmdagent.exe is what is loading the unencrypted signatures, which aren’t an issue. So wouldn’t be involved in your current issue.

I really have no idea where you are getting all these assumptions from:

At this point, all I can say definitively is something with Avast's virus definition update yesterday trashed my PC. I had no problems prior to yesterday.

I don’t know how anything can be definite based on what you have said. Since avast usually updates the signatures twice a day. That is certainly going to coincide with any problem that all of a sudden starts, what it doesn’t do is prove that that was the problem.

I have MBAM Pro that updates several times a day, if I happen to have a problem on my system today that doesn’t mean it is MBAMs fault, the same is true with avast updates. I dare say that defence+ under the control of cmdagent.exe may well be getting signature updates too.