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Ahh, the insulation…. It was like that when I moved in this past summer. I am speculating it used to have asbestos wrap and after remediation they replaced with fiberglass on the straight sections only.
No hammering the past several days so I’ll see if I can localize it more next time it happens.
FWIW this is the only photo I have of the leaky pipe layout prior to their replacement.

Following up, I have a workable solution for now. If I really choke down the garage baseboard, I can keep the 1st floor living and 2nd Floor west loops from reversing. It's not ideal because the garage baseboard isn't heating up nearly as much as before but for now it won't freeze up and everything else works properly.

The baseboard in the garage is cast iron and made by slant fin. I think it's their "Rhino Cast". I found some install instructions and it looks like the supply and returns should be on the bottom fitting on opposite sides from each other, although I am not sure if it would work just a well going into one top fitting and out the bottom fitting on the opposite side. The bleeder valve goes on the return end upper fitting of course.

Anyways, I appreciate all the help so far. I'm going to eventually make the changes to the mixing valve and rework the garage piping. I'll probably lower the radiator to the ground at the same time. I do think that my idea of cutting the garage return pipe off before the tee with the radiant floor return and connecting it in with 1/2 piping to the return stack at the bottom will improve / solve the issue. If not then it's time for check valves. Replumbing the supply and return stacks to a larger diameter 1 1/4 isn't a practical solution.

PS, Naviclean is clean and unblocked; I checked it.