Welcome to the site, hopefully i can start off your experience with some helpful advice!! For a 12 team league, especially a PPR, i really like your team a lot. Charles and Sproles are PPR gold this year and could both wind up being top 10 RBs this year so you are rock solid at the running back position. Lamar Miller in your flex could be nice if he works out, Tate is a nice backup, and I really like Woodhead in a PPR league as well to be a RB5. Overall i would give your RBs a solid A.
I like both Cobb and Bowe as well and both should catch a high volume of passes so i can see them both being in the top 12 by the end of the year, again you have 2 WR1s and when paired with 2 RB1s that means you have very nice skill positions. Not a huge fan of Nicks as my backup but with Tate and Givens behind him you have some potential upside. Again i would give you a solid A for your receivers
RG3 and Finley are both high risk high upside guys so they could be propel you into the unbeatable category or really put a damper on your team. overall though i think you did well at these positions and would probabyl give you around a B-. Given the overall construction of your team i would rate it at a B+ or A-; very solid for a 12 man league.
As far as the trade goes I like the Nicks and Tate for Jordy Nelson deal, mostly because Im high on Jordy and low on Nicks. Nicks seems to have lost a step and I think if Jordy's injuries arent nagging he can still be a top 10-15 receiver. Only reason i dont like this deal for you is because it leaves you with Cobb, Jordy and Finley and i dont like having 3 guys from the same team.
I know the second piece of the trade addresses that issue but Im not sure if i like the second piece of that trade. On the surface that trade isnt bad and I love Jimmy in a PPR league, but by making both of these trades you are completely getting rid of all of the depth you have. Essentially you are trading Miller, Nicks, Finley, and Tate for Jimmy Graham; and usually i love trading multiple mediocre guys for a stud but that might be too much in a 12 team league with little on your waiver wire.
That would only really leave you with Givens, Tate, and Woodhead to fill your flex with and be your backups (plus whatever you pick up to fill those empty spots. Im not sure there is a guy in taht group that i really want to be forced to start every single week and if one of your starters got injured you would be REALLY stretching. If you can get back another flier of a WR with Jordy in that first deal and maybe a guy like Bradshaw in the Graham deal to give you a bit more depth i might like it more