I think that it's supposed to be technically [i]possible[/i], but the lore suggests that the warlocks have been the only ones curious/interested enough to actively pursue the knowledge and practice it. I don't personally like the "warlocks are the only smart guardians" thing, which seems like a prevalent theme in the lore, I'm just stating what the game's lore seems to suggest.

Probably. I think any Guardians can do what Thanatonauts do, but the only difference is that Thanatonauts actively practice the art of dying and getting visions from their past. (For instance, there's an Exo guardian that did the same thing as Thanatonauts did, but he wasn't a Thanatonaut.)

It's more so an area of philosophy than any kind of Order or organization. Anyone can do what Thanatonauts do, which is die. What makes a Thanatonaut is looking for meaning in what you experience in the incorporeal.