‘Who is this “Chasidic Rebbe”, Abraham Joshua Heshel?’
No. He is widely respected. But it is up for him to publish his opinion if he wishes to.
” I think charlie meant a detail within Gilgulim, not whether or not they exist at all.”
Correct. The machloket was associated with the famous talking fish incident over a decades ago; this rabbi held contrary to Arizal that humans can’t return as anything other than a human.