thanks very much Andrii. you have educated me greatly. my workaround is successful and i'm able to schedule my scripts in the same way you are scheduling the post reboot exe call. i feel like i now have a much greater understading of what is happening. i still can't explain why my script doesn't work the context of running through your executable, but i'm willing to let that go. your help is appreciated.
my belief and my reading of the documentation on why the /z doesn't cause the task to be removed automatically is that it is still scheduled to run. /z says to delete it "after its final run." when it is scheduled to run on system startup, that means it hasn't hit its final run, so it doesn't self delete, it remains scheduled for the next system startup. you may be saying that you think /v1 should mean it's scheduled to run only once. if so, that doesn't seem to be the case - looks like it's for backward compatiblity with older versions of windows.
anyway, thanks very much. if you do find something about what is unique in that context when executed from customize-guest.exe, i'd be interested to hear. i am ok for my purposes now, though.