In an alternate script they were originally going to kill off Dawn and not Buffy. And in the script directions it describes Dawn being in the portal and then visibly turning back into the pure green energy that closed the portal.
So we can presume that even if the memories of Dawn remained, they would be unable to bring her back unless they could recreate the way-too-powerful spell the monks did of compressing the energy into human from once again. But if the energy, Dawn, was used to close the portal then the energy would have been used up and so doesn't exist anymore so...they wouldn't have been able to.
I think if Dawn had died instead of Buffy, Buffy would have been just as depressed in season 6 as she had been originally. She would have lost every member of her family within the space of two months. She would feel responsible for Dawn's death, would be overcome with the guilt and would not be able to forgive herself no matter what her friends did.
I don't know whether Spuffy would have definitely happened, but it's possible. Buffy would have felt as detached as she did originally, but perhaps with Dawn dying she would have been a lot more angry and agressive towards Spike. She asked him to protect her and he failed so perhaps she would keep blaming him to relieve her own guilt.
The First Evil wouldn't have been able to begin killing of Potentials and the Watcher's Council because the thing that triggered the availability for this plan was Buffy being brought back to life after being dead long to be in a Heaven Dimension.
I know it was hard for Buffy being dragged out of Heaven, but in all honesty, I think it would have been even harder for her knowing her sister was dead, that she could have saved her and didn't.
However, if the memories of Dawn were erased things would have carried on perfectly normally. Which would have made season 5 almost non-existent and definitely pointless which would have been extremely strange and time-consuming....