I finally got a chance to finish this and....WAAAHHHH! But in a good way. You did a wonderful job here, very creative and a great exploration all the possibilities of a happy ending vs. tragic ending twisting and turning till the very end. I really wouldn't have expected the sad ending at all except for your "dire warnings" in the ANs. (I thought the dead baby in the beginning was a dream sequence too) I don't usually like read dark fics, but this is so well done I am really glad I did.
Reading the comments made me glad to see that you intended to leave the ending vague, I had interpreted it to mean that Spike waited for the dawn to dust him, and that his and Buffy's failure to connect that last time combined with his despair over finding out that the child was a representation of their twisted relationship finally pushed him over the edge.