Why was Littlefinger named Lord Paramount of The Trident and not the Freys? The Freys were now lords of the Riverlands in all but name and Littlefinger is small fry upstart. Perhaps it is clear or has been answered before but thank you.

This is actually quite typical of Tywin, trying to divide and rule. Littlefinger has the title but doesn’t have the power (for some weird worldbuilding reason) to rule the Riverlands de-facto; the Freys have the power (although they’re spreading themselves quite thin) but they don’t have the legal authority to rule the Riverlands. 

As a result, everyone remains dependent on the authority of the Lannisters in King’s Landing.