JKR didn't say Snape wasn't at Godric's Hollow she said:

It doesn't mean he wasn't at Godric's Hollow just that he wasn't under the cloak. But I don't think he was there because he was busy alerting Dumbledore about Voldemort's plan.

Other than the obvious Potters and Voldemort I think there is a very good chance Wormtail was there. He was the one that recovered Voldemort's wand.

I also believe Voldemort was wiling to spare Lily's life because Snape asked him to. Snape was always one of Voldemort's favorites, perhaps even his closest advisor. So when Snape brings news of the Prophecy, Voldemort is elated. When Snape discovers that Vodlie intends to kill Lily and James he begs Voldie to spare Lily's life. After all it was really only Harry that the Drak Lord needed to kill. Voldie being pleased with Snape's work and Snape's overall service was willing to spare the "silly girl". To Voldemort this was a small thing. But Voldemort still wanted to Harry...and James.

Snape's life debut to James is what caused Snape to act differently. He may have trusted that the Dark Lord would have spared Lily but because of the debut he was indebted to safe James life. Simply, Snape could not sit by and do nothing! This went against the magic of the bond.

This is why Snape went to Dumbledore. I don't think Dumbledore would have been too surpirsed to see Snape at his door either. Dumbledore knew that Snape was a Death Eater and that he overheard the first half of the prophecy. But, I also believe Dumbledore knew both about Snape's crush on Lily and about the life debut between James and Snape. Dubledore was waiting for the magic of the life debt to kick in.

Dumbledore believe Snape did regretted that he gave Voldemort the information about prophecy he overheard because this is the information that was directly responsible for causing Vodledort to target the Potters. Dumbledore did not doubt Snape's regret because 1) he knew of Snape's true feelings for Lily and 2) he knew the life debut would cause Snape to act differently in order to save James. Simply, Snape could not have been lying. It would have been impossible due to the magic of the bond!!

We know Hagrid brought Harry from Godric Hollow to the Dursleys. We know the events at Godric Hollow took place the night before the delivery. We know that during the delivery, Harry fell asleep over Bristol. We know that one year olds can't go for 24 hours without food and diaper changing. Where was Harry in those 24 hours and what was happening to him? The other person at Godric Hollow would know the answer. In rereading book one, when Hagrid takes Harry to the Leaky Cauldron, the bartender states, "welcome back Mr. Potter, welcome back". Was Harry there before?

The more I think about it, the less I think Peter was at Godric Hollow that night. Although he betrayed the Potters, I don't think he had the courage to face them. I think Snape and Karakov were there. They are both reformed DEs, and from the indications in GoF, are still good friends. Something drastic had to happen to both of them for this to happen. When the AK curse backfired, they had to make plans real fast. I think Snape ran to DD and Karakov picked up LVs wand, and gave it to Peter. After Sirius found and confronted Peter, Karakov returned and transformed Peter into rat form, and ran for his life. This would plant the blame on Peter and render him incapable of relating the events of the night.