We have a bit of experience with this, making large conical shelters, and heating them , and also adding liners or nests. I have not done any testing with a static BTU source but here are some non scientific findings.
When running a stove a liner adds a fair amount of warmth while the stove is running. Yes the apex can be much warmer than ground level. Sometimes in 20F the ground level of the tent can be 65 -70 while the cone is well above 100, probably closer to 120 - 130F.
A small nest with DWR on 3 sides, seems to add close to 10F in a small micro climate
Yes, condensation always forms on the outside, unless the layers touch. The same for frost
In one case, a few inches of snow fell overnight, the tent was pitched on soil, but the rest of the surrounding area was snow. The temperature was about 15F outside in the morning, but inside the tent it was above 40. I attribute this to a couple factors, mainly the inside of the tent the day before had been probably 80 F due to the sun hitting it, even though outside it was likely never above 32. The snow then provided insulation about 2.5 - 3 ft up the outside wall. The liner provided a small but second level of insulation. So, you had ground that was warm and giving off heat, and then an insulation layer of snow added around about 50% of the tent fabric. I wouldn't count on this to keep me warm and take a lighter bag, however I would be pleasantly surprised to wake up and find my tent well above freezing after a night of snow.
When it gets cold, we plan to experiment and test essentially reflective roofs near the apex, to see if they minimize heat loss where most of it is loss.
Regarding summer use, effective ventilation is the best answer. You can still use the same DWR's etc, as long as you have good venting. This is more than just apex venting. Controlled venting via doors or the base goes a long ways.