Still waiting for more details, but I found a trim electrical schematic with a slightly different layout, but may add some clarity to the schematic BDAttitude posted above at post #735, in particular the function of the Main Electric Trim Interlock. I'm not claiming to be an expert in reading wiring diagrams, but here's what I see. Flight deck inputs come from the left. The right side represents the trim motor and associated electronic controls. All the FCC inputs (MCAS, STS, Mach Trim, MCAS) appear to come in (off another schematic) into the trim motor box from the right. Various control and limit relays are in the middle.
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If I am reading this correctly the "Main Trim Arm" relay is activated by one of the pilot yoke switches which, among other things, sends a signal to and opens the interlock relay contained in the trim motor box on the right side:
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The interlock relay appears to be a basic relay and not a function of any IC processors. This relay appears to be spring-loaded closed allowing automated trim inputs by default (which makes sense because the STS and Mach Trim systems need to be active full time). When a Main Electric Trim switch is actuated, the interlock opens, and this "open" signal goes off the right hand side of the page presumably into the FCC logic which
should stop any automated inputs. There is no way to tell from this diagram how the FCC handles that input.
If for some reason the FCC logic does not process the "stop" signal, then I presume that
both the Main Electric Trim and the FCC could attempt to send trim signals to the box marked "Controller" located just above the box labeled "Brushless DC Motor." We can't really tell from this schematic if that could happen, or what the Controller would do if it received conflicting signals.
So sorry, no definitive answer, but perhaps narrows the focus some.
All of this should be qualified by the fact that this so far appears to be an issue with the
updated FCC software/firmware. Not sure if it could be extended to the original software.