To clarify the bridge question, it isn't structures the way it is because it is a test for guardians. The Book of Sorrows talks about how Crota cut a hole that (unintentionally) let the vex into the throne worlds of the hive. The hive were never able to close the cut/hole and therefore developed the annihilator totems (the 3 pads that must have at least one guardian on them at that bridge). The annihilator totems were created to kill all vex that entered the hive throne worlds. Therefore since the bridge part of the raid is in the ascendant realm/throne world, those totems that the guardians must stand on act like a first wall of defense for Crota's throne room.