This theory is beautifully stupid. Future-bearded-clone-Jack has read about the death of his time-travelled-Oceanic 815 survivor-nonclone-alternate reality-Jack (who never divorced Sarah and had a son with her and lived for a while in New York) and has gone to visit "himself" in the coffin. That is why he couldn't commit suicide, because this piece of him was already dead. That is also why he is so upset that no one else came. And Kate didn't want to go because that Jack was married to Sarah, not Kate.
All I can say to this is "wow."
Well, not completely true. I can also say "stupid!"