I think he's dead. I can't think of any reason why he or whoever flooded the image would have used an animation if he was actually alive. I think the 'real' clip at the end of the credits was just a teaser for the next series. He doesn't say 'Did you miss me?' - which would imply he's coming back. He just says "Miss me?" - which could mean anything.

I don't know who created and released it. I like the theory of it being Mycroft. Not sure he would compromise his position in the government though.
I also think it's possible that it was something Moriarty set up before he died. Moriarty had a death-wish, he was quite prepared to die. I think it's likely he would have made sure there was a fall-out after it happened.
 

I didn't think he looked sad or dejected, I thought he looked thoughful.
And he had that fantastic 'swinging his coat' on moment that I had on a 7sec replay loop ...

His work at the wedding was done. He had other things to think about, most likely the telegram that he'd read out to Mary, her reaction to that must have been one for him to store away until he had to consider it ...

"To Mary, lots of love poppet, oodles of love and heaps of good wishes, from Cam. Wish your family could have seen this..."

I agree that Sherlock is not a pressure point for John, as John would believe Sherlock capable of out-smarting anyone who tried to blackmail him using Sherlock. CAM is, Sherlock tells us, 'The Napoleon of blackmail'. I do wonder how CAM could use Redbeard to blackmail Sherlock. I have a feeling there is more to the Redbeard story than just being a family pet that was put down. CAM he has a 'file' on it. When he says 'Redbeard', Sherlock looks shaken and stutters.

I'm also intrigued as to what the Porn Preference means. Elizabeth Smallwood does have a porn preference, it is 'None'. I don't know why Sherlocks isn't None too. There's obviously some distinction between Normal and None if CAM bothers to differentiate them. Smallwood also had heading Vices: None, which I don't remember seeing for anyone else. I don't know if maybe CAM just searches until he finds what he can use ... And I don't know why Sherlocks pressure points were listed over and over again.

For Johns reaction to Sherlock going away, I'd guess he was trying to protect himself. He'd already greived for Sherlock for two years. I would imagine knowing he was going to lose him again he would try and distance himself as far as he could as he knows what he death had done to him before.   I found his the scene where he 'spoke' to him in the graveyard incredibly emotional. He knew Sherlock had heard that, what else would he have left to say?