This is a very long and in depth theory, well done. As to why the vex don't travel backwards in time and end us, that's is a very difficult question involving time travel and a higher plane of thought, both of which us humans can begin to comprehend. Maybe they're only reactionary, as I think you stated. Now I can remember, did we actually defeat the vex? Or just another branch of it? Because it did that with the hive, you kill that shrine of Oryx I believe? And your ghost says it was communicating with something bigger out there, their Hive god/king Bungie left a lot of loose ends open for use to deal with in the future.