I personally think that they randomize the order that exotics will appear and then plan for xur to sell those. This means that they can still change it and see what he will have. This explains both deej's fvck up and datamined info.

Sure, but the minute they choose to change it, it isnt random. I say all that to say this... they will keep the gally until it serves them to have xur sell it. My best guess is around mid summer , but i will admit its only a guess