As far as the witnesses of Godrics Hollow, one thing we do know is that Dumbledore was very clear on what went down there.. he seemed to know an awful lot.
Could Godric's Hollow have actually been Dumbledore's house, that he offered the Potters as protection? Since it is a very good chance that Dumbledore was Gryffindors heir, it would make sense for him to live in Godric's Hollow. I can't remember whether that was the name of a street, or an individual house or whatever though.
Perhaps he learnt what happened through legilimency, and obtaining memories, as we have seen him do before - and if that is the case, maybe there were witnesses that we don't know about yet. The death eaters were almost certainly there, even ifr they were told to wait outside - could Snape have told him? Either way, i think we can be fairly sure of what happened there. Voldemort went in, killed James, killed Lily, tried to kill Harry and failed. The only evidence to oppose this is your idea that Voldemort is lying - and it's true, he is a liar - but it's still just a theory, whereas we have been presented with evidence of what happened, whether through Dumbledore, through Harry's flashbacks, or, whether for good or bad, Voldy himself.
Every time i see the argument that it wasn't Voldemorts intention to kill Harry, or that he intentionally wanted to make him a Horcrux... it just seems so optimistic, so unlikely. The argument is that Voldmeort went to Godrics Hollow with the intention of making Harry a horcrux, not to kill him - but now, he has changed his mind and wants him dead? He knew of the prophecy before he attempted Harry's murder; knew that this child has the power to defeat him. We are lead to believe that he went there to kill him, but you say Volde was thinking 'ah, if i put this bit of soul in him, he wont be able to kill me, because A, i have a horcrux in him, so i can rise again, and B, he will have to die before i can die.' The flaw in this is that Voldemort cast a killing curse at Harry. You wouldn't devise such a crafty plan just to go and kill him.
Throughout the first 5 books, Voldemort has wanted Harry killed. The only time we know this has changed, is in HBP, when Snape reminds the death eaters that "Potter belongs to the Dark Lord - we are to leave him!"
This quote can be taken a number of ways. Indeed, perhaps Voldemort, by now, has actually discovered that by some freak of nature he made Harry a horcrux (he was inside his body in the ministry - did he realise, when he possessed him in OoTP?) Perhaps he simply wants to be the one who kills him once and for all? We have seen him say pretty much the same thing in GoF, when it is clear he wants Harry dead: "Do nothing!"
But even as i write this post, i noticed that indivudual word.. "Potter belongs to the Dark Lord". I don't even subscribe to this theory that Harry is a Horcrux - but that word 'belongs' can have many meanings, and is an interesting choice, no? People don't belong to other people. Items belong to other people. Horcruxes.. belong to other people?