I think it might be possible, Minty. Another poster earlier in the thread related a story where her daughter went missing for a short while, and she had no idea at all how long time had passed for. It's not like you're checking your watch the whole time. I think they could have got it wrong, not for sinister reasons or anything, just it's easy to lose track of time when you're in a different place, it's relaxed, you're relaxed, and then you realise something is wrong - I doubt anyone confirmed what time they last saw him and when they started looking? It may very well have felt like 1-5 minutes, but that doesn't mean it was.
Certainly, it can be confirmed that it's absolutely no earlier than the time on the photo of WT as Spiderman on the deck. So anytime between then and when the cops were called, depending on when dad left - whether that was before or after the Spidey photo?
Grandma falling asleep! That's so possible, if she's been unwell. She might not even have really realised she fell asleep, or for how long for, depending on how she is. Good thinking Minty. Poor Grandma. I hope she's doing okay.