Personally I say that there is no such thing as a waterproof theory (i should know - my raincoat leaks) every theory has holes in it. And even the most bizarre theory has a chance of being right. For example saying that the moon is made of cheese...I'm sure that somewhere in the universe there is a race whose word for rock is "cheese"... or that the Smallest asteroid in the universe is made of blackcurrent jam and speaks kansai-ban japanese
Me i don't buy into the theory that Bosch was really an uber-deek cover operative. I do believe that something fishy was going on. I mean for all we know Bosch may have had supporters in high-level positions within the GTVA - supporters who believed in his dream of allying with the shivans...For all we know there was some "X-files type" conspiricy going on in the GTVA
Given that we know almost nothing about the shivans and that so many questions were left unanswered, just about any theory we could come up with is correct. We know NOTHING about

's plans for what would be resolved in that-game-which-i-dare-not-mention, other than vague comments from them. So just about any theory we could come up with stands a chance of being correct. Of course some stand more chance than others...
I myself have several theories
1) The shivans serve the same purpose in the universe as Gort from The Day the Earth Stood Still
2) The Shivans are heralds
of Galactus of some even more advanced race or alliance who make contact with younger races based on how they deal with the shivans - ie they make contact with races capable of defeating or negotiating with the shivans.
3)
The Shivan's Enemy Theory 4)
The Shivans Insulted TheoryOut of those 4 theories, i personally find 1) and 3) the most likely.
2) is possible. Of course i have to define what i mean by "Contact" ( i vame up with 2) as i was typing)
4) is not very likely but still within the realms of possibility.