OS X _is_ a real UNIX, plus a lot, lot more.
Uh, you mean a lot less, don't you?
The python (and wish version) of the scripts run fine when invoked from X11 locally. When someone ssh's into the machine from the X11.app, regardless of ssh args, and invokes the script the same way, the script will only display on the machine that's executing the script, not the user's machine.
I had thought that the problem was between scripts/compiled programs, but the compiled program (FlashNet backup/archive) software had installed it's own X11 library in /usr/local/lib. I just found it this past week.
Try reversing your experiment. Put the script on the Mac mini and get it to display on the Ubuntu box. It'll work fine with the script running on the Linux box and displaying on the mac, but when the mac executes the script, the wheels fall off.
I'm not aware of having to call a window manager explicitly in a Python script. TBH, I thought that that was handled when I imported Tkinter:
from Tkinter import *