LOL Colombo. I can imagine many a pub discussion as to whether the child actually died or not...
I agree that there are loose ends... but then perhaps that's the point? Real life dramas don't usually work out as neatly as 58-minute detective episodes with every aspect made evident.
Aside from the fate of the little boy, I find the contrasting portrayal of the parents to be quite poignant.
Both are distraught by grief, but they have very different personalities and deal with the tragedy in different ways.
On the surface, Emily (a quieter, more private personality) appears to accept that her child is dead and attempts to move on with her life (with the loving support of others), but in fact is repressing the grief deep down inside. She can't cope with any more false hopes, but gradually accepts that Tony may indeed have found something. At the end, she seems to find closure in the explanation given.
Tony has a more volatile and combative personality and his behaviour in the face of that grief isolates him, partly of his own making. He won't give up, however tenuous the lead may be. His perseverance did indeed unravel some answers, but leaves a number of questions.
Has his grief made him go mad, or is there still a possibility that Ollie could still be alive somewhere?
Emily saw an apparition on her wedding day, but seemed to accept that he was there with her in spirit and no longer as the physical manifestation of her little boy.
Did Tony etch that drawing on the frosty window himself, as a manifestation of his denial, or not?