I guess it still comes down to whether this is an issue in tkinter or in Tk itself.  Almost all character processing and event generation is done in Tk.  One way to isolate the issue would be to use text widgets in the Tk wish shell and its demo programs.  They should be available in the ActiveState ActiveTcl distribution.  (I'm assuming that the Python Windows installer does not include wish but I don't know that for a fact.)