Proud member of the "Who cares about no beta, Cataclysm's out now" club.
What kind of reason does Garrosh have to get Thrall captured? He already Warchief even before Thrall goes away he doesnt take it after Thrall was captured so. Besides him giving that SI:7 insignea is just to show: The Alliance have tryed to capture me or kill its war time or something here is also a quest text from Thrall as you save him:
If not for you, I would surely be on my way back to Stormwind to be paraded as a trophy and a slave.
The Alliance dogs found me unconscious on the shore and put me in this arcane cage. It was keeping me from connecting with the elements.
No longer!
/blink
You haven't actully presented any evidense to back your counter claim that Garrosh was not involved in Thralls capture. Far from it, the released quest text showed how Garrosh went pale at the news of Thrall being alive.