Messages from Ableton are still ambiguous as to the touchstrip issue and how it will ultimately be resolved. I doubt I will gather any more solid clues via further testing. Although certain patterns point to a hardware issue, it is always possible that even if that is the case, they will be able to work round it in firmware. Or maybe not, will just have to wait and see.
I did have a go at seeing whether any of the sysex commands to change touchstrip configuration (these are the same on Push 3 as they were for Push 2 which has documentation for this on github) could get the touchstrip to behave normally again once the problem had begun. Nothing I tried made any difference.
I suppose I will do one final sort of test, whether the problem is ever triggered in controller mode if I dont even run Ableton Live when in that mode. But it might take me days to reach a conclusion via this test, because it seems like my unit might be much less likely to suffer the issue once its been on for ages.