For now everyone forgot Sai. He's now up for his super important secret mission. Is it related to Akatsuki? Is it related to Kyubi? Or is it some sort of mission old man entrusted him with? For now he's just watching, but at some point he's definately gonna step in.

As for seal 4th hokage seal weakining - I don't think so this is really a case. Since Kyubi watches and knows everything about Naruto, Kyubi learned how to pull some strings on Naruto. As we all saw how he manipulated Naruto into entering his prison. Thats all there is.