On the lack of a public timeline on #mastodon instances, I think this will sort itself out over time. Either they'll add that as a feature, or users can just hop randomly between instances until they find one they like, plus recommendations via gossip are always possible.

Not having a public timeline is privacy via obscurity and therefore the very weakest form of privacy you can get. It maybe deters the lowest hanging most knuckle-dragging of adversaries, but not much else.