While I don't disagree that I must be careful as to rate of climb, my feeling is that as long as I climb no faster than the normal rate of rise of the balloon I should be OK. Again, the balloon tether will be run thru a lightweight, rigid tube for the first 15" or so to try and keep the balloon away from the props if the worst happens. My gut feel is that this will not be an issue.I reckon that, on take off, the prop down-wash will draw in the balloon, and that'll be the end of the experiment..
Gravity will work in opposite directions on the upper and lower craft. The lower (my P3P) will be pulled toward the earth by gravity, the upper (the balloon) will be -in effect- pushed up by gravity pulling on the denser air and "floating" the balloon upward. Never the twain shall meet.