For instance, if the Ghost of Christmas Yet to Come is capable of seeing the future, then why didn't it show Scrooge's impact as a new person?
Probably because Dickens realized that would make for terrible storytelling.
Showing Scrooge his negative future seems much more logical that showing him the positive one, if Scrooge is too learn about the need for him to change his ways. Show Scrooge the positive future and he'd assume he could continue on leading his life as he had been. The spirit would have had to verbally "set this up" for Scrooge to understand he was viewing a positive future that required him to change his ways. As written, the spirit didn't speak a word -- showing Scrooge his negative future was much simpler and effective storytelling.
Why did Tiny Tim die in that vision if he didn't actually die so young as prophesized? If the GoCYtC is a conduit to the future, then it is a faulty conduit, because it failed to show Scrooge what was indeed going to transpire.
Because the vision of the future shown was what would have happen if Scrooge hadn't changed his ways.
But the GoCYtC is not a conduit to the future, it's a determinant of the future ...
According to whom?