Mountain Town is located “about halfway between Port Mary and the interior mountains”, so it also may connect there, even if it’s blocked off with the cave-in. So it makes sense for MT to connect via Blackrock to Port Mary. It’s hard to see how that’ll work - maybe MT to Pleasant Valley or Keeper’s Pass. Then that would plausibly connect it to BR and PM.

Edit: Keeper’s Pass South has a collapsed tunnel that definitely leads to MT, so that answers that. So the Port Mary connection probably happens via Pleasant Valley, past Thompson’s Crossing through the blocked-off curve there.

Oh, and we know Mountain Town is supposed to connect to Blackrock somehow. It seems like they’d need a pretty substantial transition region between those two to justify it.