Q: Repeated prompts for Appleid Password on MBP: malware?

Possibly… I'll check. (As for Facetime, it's disabled on all devices, but I'll check other background stuff.)

Thanks: very useful to know. However, FWIW, let's say I have taken the insecure path and used the same password for appleid everywhere—iCloud, iTunes, all apps, and on, and on forever, world without end. And let's say I have  *one time* entered the password for each and then never again in that session (in a manner of which I am aware) "logged off". Should, after reboot, there *still* be any further prompts for passwords (app-specific or not)?

One might think so, but I am never asked to store passwords in the keychain. I will look to see whether (1) I have keychain storage of passwords—somehow—disabled and (2) these passwords are (not) there. Thanks.

(Tried to mark your post Useful, both in my Email response and here: couldn't figure out how!)

Thanks, Eric. "First Aid" check cited no problems. So I've bookmarked the Reset instructions page you cited, and will go through it when I get desperate and/or angry enough. Thanks again.