Responses to above in no particular order:
1. The stab trim motor has four speeds - Main Electric fast/slow (0.4/0.2 deg/sec) and Automatic fast/slow (0.27/0.09 deg/sec) and two directions. If the previously posted schematics are to be taken at face value, then the commands for speed and direction come from outside the motor unit, but the actual processing of those commands into a specific speed and direction occur within a controller housed within the motor unit. While no documentation regarding this controller and how it operates has been publicly released, it is reasonable to assume that the accident investigating bodies would have access to it if they felt it was needed.
2. Overheating. This was a problem on early Boeing designs, but I am not aware of any such issues since introduction of the NG, and there is nothing is the manuals related to a thermal relief circuit. However, if there was an undocumented thermal relief, then it would temporarily disable the motor completely, and not just in one direction. No signs of this occurring. Again, if there was any history of this being an issue, one of the hundreds (or perhaps thousands?) of interested parties would have pointed to it by now.
3. Trimming forces. Speaking as someone who has actually used the manual trim wheel on the Boeing, I can assure you that the trim resistance increases in both directions. It is not like rolling a boulder uphill where it is very difficult in one direction and easy in the other. The increased resistance is due to binding forces at the jackscrew thread/nut interface. Not sure if it is strictly frictional force or some type of deformation or misalignment. Keep in mind that while the aerodynamic forces are acting roughly parallel to the jackscrew body, those forces are mostly perpendicular at the thread/nut interface. There will be some directional bias, but that is a relatively small component. BTW, this is one of the reasons why a jackscrew mechanism is used in the first place - to minimize those directional forces and make it easier for the motor. However, one point needs to be emphasized again: You completely avoid these issues by not letting the stab get out of trim in the first place. There is no, none, not one iota of evidence that the Main Electric Trim did not move the stab when it was used. And yes, by not being attentive to such matters, it is absolutely possible to place the aircraft in a state where things don't work as they should.
4. There is a lot of reference of what are perceived as anomalies in the trim system in the final moments of both accidents. Personally, that's not what I see, however I'm not going to claim I have a better microscope than anyone else here. That being said, I must again point out that in those final moments, the aircraft had exceeded the certified flight envelope. Engineering safety factors aside, there is not any expressed or implied guarantee that any system will work as it is supposed to once you exceed the limits of that system. The lesson here is that you keep that aircraft within its design envelope - just like the crew of the penultimate Lion Air 610 flight did.
5. There is far too much post hoc reasoning going on here. If your going-in assumption is, "Of course the Main Electric Trim must have malfunctioned," then your mind is going to work very hard to disregard any contrary evidence. As the initial data started coming in, one of the first and most obvious question to arise was whether there was a problem with the pilots' ability to trim the aircraft to oppose the MCAS input. That was certainly a concern of mine. I actually operate a 737 on a regular basis, and I have a keen interest in knowing. However, there is a big difference between asking if there was a malfunction in the pilot's trim system and assuming there was. In the first case, you go where the evidence leads, in the latter, you try to shoehorn the evidence into your assumptions.
6. Corollary to point 5. Whether or not the pilots had the tools to oppose the MCAS input is one of the top issues that the various accident investigators and certificate authorities must have looked at. Consider all the public discussions regarding the issues with the manual trim system. Consider the discussions regarding adequate pilot response time and whether the Runaway Stab Trim system checklist should be re-written. Concerns regarding these two items may, in fact, be some of the issues slowing down the process of getting the MAX back in service. Compare that to the absolute lack of any comments by any authoritative body that there was a problem with the Main Electric Trim system on the accident aircraft. You just can't assume that all of these parties with all of their vast resources are either that incompetent or part of some conspiracy to hide this information. This is the case of the dog that did not bark. It is not barking, because there is nothing there.