Could the early Church have survived and thrived without the authoritarian elements of what we now call Catholicism
I still have to see how orthodox Christianity was more authoritarian and monomaniac than the other branchs (Donatists are quite fantastics on their puritanism, for exemple).
Orthodoxy built itself not only on its own doctrianism, but as well defining against other branchs. Nature of the Christ wasn't that defined doctrinally before Arius showed up, and so on.
The early church, far from the myth of a pure spirituality corrupted later, was a mess of contradicting and blurry conception that were refinded to theological and political conflicts, giving birth to several denominations (including orthodoxy/catholicism)
It wouldn't take that much to have arianism or homeism dominating in the period of establishment as imperial institution (afterwards, when orthodoxy becomes...well, orthodoxy and hugely widespread among Romans, it's a bit too late) for instance.
Gnostic cults would have a far harder time, mostly because it was concieved as an elit cult, based on the enlightement of a few.
Judeo-Christianism or Judeo-Nazareism was a bit too foreign to really fit Roman reality. I could see last in Orient and expanding into Arabia (where it arguably develloped quite a bit), but never made it to Roman acculturation.
The moment it made it to this, as a roman institution where the emperor had the last say in religious matters? It was bound to get a hierarchical structure. Giving that Christianism would evolve in a roman world, and that a successful Christianism would have to fit this world, it's more or less impossible to dodge.