as you said it is very difficult to ascertain he "truth" of the
matter with respect to what is the original mantra - hare krishna,
hare rama or both. hare rama is chanted widely in the smartha
tradition. though adi sankara taught the importance of chanting the
holy name incuding harer namaiva kevalam, he did not teach this
mahamantra. it is possible it was passed down in the privacy of guru -
sishya relationship as it is a vedic mantra. now the question of who
popularized the hare rama mantra remains unanswered. one possibility
is caitanya taught this during the south indian tour. but the opinion
of the gaudiya acharyas is that caitanya chanted hare krishna
mahamantra. so caitanya's role in preaching hare rama mantra can be
eliminated by finding reference to hare krishna mahamantra in the
contemporary works of caitanya. is there any such reference ? even if
we eliminate caianya as the preacher of hare rama mantra, one
important question remains unanswered : who is the preacher of hare
rama mantra, when did he live and why was it preached that way ? if
he is not caitanya.

one way to ascertain is to look for original palmscript manuscripts
of the kalisantarana upanishad to see if he has either one or both.
those who wrote critical editions would have looked at these
manuscripts and preserved it. if the manuscripts were not available,
they would preserved interview records with the proponents. i am
looking for that data as the question is fundamental to the practice
of devotees all over the world.