At this time Panama is still part of Columbia, US does as OTL and supports a Panamanian revolution and the canal is now in a different country that demands their sovreign territory back, if France refuses the US helps Panama take it
In any case Panama cannot run a canal on their own and will need to make an agreement with the US to run it, that just happens to give the USA de facto ownership of the Canal
Why do you assume automatically that it would be the US that back such a regime., If the French choose to back either party (the one which preserves their interests best) what would the US reasonably do......Nothing I tell you... the whole Manifest Destiny, Monroe doctrine Roosevelt corollary thing is past its time and honestly... was never worth the paper that anyone printed it on... It was all a bunch of hot air...smoke and mirrors