re-reading this whole thread
1) still think if it came this way from the builder it is the builders issue
2) something is very off
3) lot's of assumptions are being made without accurate measurements
4) the idea that the seat tube is tapered is curious if accurae
IMHO nothing should be done until detail ID measurement are made the full seatpost insertion length, using real measurement tools this will tell a better picture of what is reality is
once that is known then a solution can be determined
1) go back to builder with facts and see what response is
2) get a machinist to build a seatpost shim to fit size and go for smaller post size like 27.2 using the shim (something like the problem solvers)
https://www.universalcycles.com/shop...ils.php?id=914
2.1) ream to a consistent size if needed but from pics does not appear to be a log of material in the current insert
3) try to shim with sheet material
I personally think number 2 would be best assuming as stated builder is doing nothing