Hey there Tnsqwatawa (that's quite a mouthful

). It would seem to me that there is no sure-fire method for preserving an in-ground stump over an extended period of time. Even if you take severe precautions they would only affect the visible portions of your work. I would imagine that weathering and decay from within and underneath would be very difficult to overcome.
Of course, I may be completely wrong as I have never attempted to do what you propose and someone in here may have some more helpful insight. The sealing, varnishing and termite protection you have so far seem like excellent ideas. I wonder if they would be deterrents to ants and other wood loving insects though.
This would actually make for a wonderful case studyand maybe some others in here have other precautionary ideas or have stumps of their own to try this stuff on. It would certainly seem helpful if we were to know one way or the other.
Good Luck and welcome to the forums,
CM