With multiple busdrivers, you'd establish a chain. Person A would take the damage meant for the person they'd been forced to Obstruct, but that damage would then be passed along to Person B. So for the purposes of effects, Person A is still Constricted (and i.e., the effect would fail if they were nullified), but for the purposes of damage Person B would still end up taking one for the team.
And Reversal counts as hostile for the purposes of Obstruct/Constrict.
Last Gasp is still treated as a group power, and so the player that triggers it doesn't have to be Specialist to use Assassinate or Psychic to use Mindblast. Either can be used regardless of sub-faction.