Hi Robert, External air has been discussed a fair amount here, having its pros and cons but typically favouring the cons. The best place I found that covers it well is here: the-outdoor-air-myth-exposed
In the RMH world, a chimney fan is only necessary (for some people) when you have poor draft at startup with a cold mass and want to give a little boost at the beginning of a burn, then turned off.
A bypass is a more low -tech solution that is hard to beat for its simplicity and reliability.
I'll take a try answering your questions.
#1) Using an earth tube to supply your Primary air.
I can see no benefit from this. I'm not sure how one would plumb such a thing. But my concern would be volume.
It takes a lot of air for an 8" J tube.
What were your thoughts about this? What was your idea?
#2) Using a chimney fan is it a good idea?
Well the answer is Yes and No ! A fan is sometimes necessary to start the draft until your chimney heats up.
Running one all the time is not a good idea.
Designing and installing a bypass gate to preheat your chimney is a much better way to kick start a dragon... no electricity needed!