I see what you mean and I respect everyone's opinion on this but I personally think that is supposed to be Haruto. Even though it seems a bit rushed for Haruto to be with another woman and have a child already, I think that the director/writer wanted to show that he went on living, it's what Aya's message has been all along, right? Even at the end she said 'Ikirunda' and 'Ikiru' means 'live' so she said something like 'Live on'. Also, I don't think it said anywhere how much time elapsed after Aya's death that the parent's visited her grave. It could have been soon after, a year later or a few years later even. I don't think it's impossible for Haruto to have a kid. Also, if you think about it, Madoka and Tanaka-sensei also had a child and to me he looks between 4-6 years old? Anyway, we can all just ponder about this.
As for behind-the-scenes or outtakes. I saw some extras but they were just quick interviews but no Ryo sadly.
that could be but i think like what other people said, aya's parents and her docter would have acknowledged that he was Haruto because Haruto had as big a role as the docter. plus whats the point of putting Haruto in if he moves on already, only a year after.
the ending with all those people are suppose to represent the fact that aya's diary affected a lot of people and those people appreciate and are coming to pay respect to her. since they don't show anyone specific, i think they are people that don't personally know the family.