I'm inclined to agree with you about Luck. It would be awesome to have him for years to come in a keeper league. I really don't like the downgrade from Decker to Johnson, but Luck seems to be much more of a sure thing than Newton. I also really like the idea of getting Amendola and FJax for Decker, especially in a PPR, but if you do the Luck trade you obviously can't do that one.
I like your plan of doing trades 1 and 3, but I would try to squeeze the Cobb owner for a little more. I think you could get better than Cobb for a QB like Newton, even with Cobb's success and Newton's struggles. But if you can't, Cobb will be awesome with Rodgers throwing to him for the foreseeable future. If you take trades 1 and 3 you'll essentially have upgraded at QB and washed out at WR (Cobb=Decker in PPR). I like that plan personally.