If i knew you were still insisting on this non-sense, despite so many data points pointing to the contrary (and none that support your own, except apparently the experience of yourself alone), I would've just taken screenshots for my last flight, as it was another one where award inventory was not available but upgrade was possible. If your claim was true, that should not be possible, since you insist award and upgrade inventory both come from I.

If another FTer is doing a Z upgrade anytime soon (when I inventory is not available), could he/she please take a screenshot of the award screen showing no availability, and then a screenshot of the upgrade screen showing upgrade is available (I know this is easily done if using Aeroplan, no personal experience with United) to settle this debate once and for all?