1.) You appear to be on to something here, though, I am not sure if its the whole thing. When I did a test print with the fan turned off the under extrusion seems to be reduced. Still present, but reduced. It occurs to me that a difference between the working UM1 at home and this one is that I printed new fan shroud for that one. Is there one you would recommend specifically?
2.) I just replaced the thermocouple so it seems unlikely. If it is the thermocouple it is at the very least consistent. I have watched the temp reading during the print and it never varies more than a degree (Except when the fan kicks on but that is quickly recovered).
How can I tell if it is boiling? Just heat it up and listen and watch for bubbles in the filament as it oozes out of the nozzle?
3.) I have never printed ABS, and as I mentioned the nozzle, brass insert and peak tube are all brand new from UM, so clogging in there from gunk seems unlikely.
4.) I will have to gather some equipment to perform the test you suggested, but I will say that in the past when cleaning the nozzle I haven't ever found any wood debris.