...for that matter, why the urgency on desmond's part in the sideways universe to awaken everyone else? if all they are doing is realizing they are dead, remembering the island and each other and letting go, then what's the rush? and what does desmond get out of helping them?

charlie helping desmond, that makes sense. charlie's vision of the island life was incomplete--he didn't fully awaken until aaron was born--so he just knew something was up and wanted to share

desmond's actions, the plot of waking people to their previous lives--that fits better with the sideways tying into the plotline on the island, not with the sideways being some afterthought we got to see early