Yeah, but we are all speculating. And, I find it strange the Rowling would go to the lengths to let us believe the carriages were horseless then reveal that they aren't. Going so far as to create an animal that can only be seen by someone who has seen a person be snuffed out. Then have Harry freak out so badly when he finally sees them in OOTP after Cedric's death. I mean we can all put IMO at the end of all of these posts...
Of COURSE we're speculating! To me, that's the magic (pun intended) of the Harry Potter series. Not only has JKR sparked an interest in children to read, a lot of (supposedly sensible) adults on this thread and another I'm following are having fun with what to happen to fictional characters.
It's not great literature. But it is a fun read.
Now, something else to consider:
I know we've all read about the great bond between Lily and James, but could it be possible that a tiff will be revealed between them, that Snape found Lily heartbroken and they, um... consoled each other.
Then Lily and James patched thing up, but Harry is REALLY Snape's son? And he's been so awful to him because Harry had to go to Muggles because Snape couldn't claim him?
Hmmmm....
Keep in mind that Harry was only one year old when his parents were killed. Perhaps he didn't actually SEE it happen. Maybe he was wrapped up in a blanket? Maybe his mother was holding his head to her shoulder? Maybe he was in his crib in another room? His recollections of the time when they died only include hearing his mother scream with no associated visual images.
As you point out, though, it's all speculation. I just have a hard time believing they could still be alive when everyone else -- including Dumbledore -- acknowledges that they were killed.