How many flags are allowed to be on one pole?
Is a flag ever flown up side down at half-mast, more than 1 and why?
A flag is flown at half-mast in memoriam as long as deemed appropriate by the head-of-state of the sovereignty. Note that in U.S. state governors are deemed to be sovereign within their states in this regard. (The precise delineation between Gubernatorial and Presidential authority in this regard is a Constitutional Law issue on which SCOTUS hasn't ruled on yet (to the best of my knowledge).)
For the death of a particularly important personage of a sovereignty it is traditional to fly the flag at half-mast until internment. This was, for example if perhaps reluctantly, the case for the death last summer of John McCain.
President Trump belatedly issued a statement praising McCain's service to the country, and signed a proclamation ordering flags to be flown at half-staff until McCain's interment