> compiles _tkinter is such a way that it looks for the two dlls 'everywhere' (in pcbuild itself and 5-10 other, non-existent directories)

I think you're confusing the finding of the tcl/tk DLLs with the finding of init.tcl; the DLLs are searched for on PATH (as I understand it, just like any other DLL), while init.tcl is searched for in several places hard coded deep in bowels of Tcl (see #20035).

Python 3.5 (default branch) builds Tcl/Tk as part of the build solution rather than as part of external*.bat, and copies the DLLs to the output directory as part of the new system.  3.4 and 2.7 could be fixed to copy the DLLs as part of external*.bat, but will have issues similar to #21059 without some kind of fix like #20035 (which is currently only targetting default branch).