The whole "left and took his stuff" bit is exceedingly unlikely and made popular by an "insider" on Reddit. You don't own the rights to what you write under hire unless previously agreed otherwise by all parties involved. I think it's highly unlikely that before starting to work on Destiny Bungie would have given Staten the rights to Destiny's scripts. That would be extremely unorthodox, though admittedly not impossible.
As for Grimoire being written late by a random guy and having no connection to anything else...again, Reddit rumor levels of salt grains required. Possible of course, but not something I'd feel comfortable taking as a fact. A rumor making sense != being true.
Something definitely went horribly wrong in the game's development but I'm fairly certain it wasn't a case of the lead writer leaving and Bungie going "shit we don't have a story anymore!". It was probably more along the lines of the story being outside the scope of what they were capable of giving us, or not meshing with their overall gameplay vision for the campaign.
Whatever happened it's a damn shame to be honest.
