Great tread here folks, wish I had time to read it all!
Just on another idea, why not just adapt something already made, so we don´t reinvent the wheel, like Anjin said above on the welders?
I for one have done that, I took this all-metal extruder from China and made some parts to fit the um2, to give it the right tension, etc, and it´s working like a charm now! The fact of the feeder being all metal is really a plus, and it´s not expensive at all, and you get the increased functionality of having an easy material changing mechanism (just pull the lever up and take the filament out).
If someone interested, I can take pictures and upload on youmagine, etc...
Cheers!