I think the spirited away friend is actually Yukito. He calls him Yuu in his flashback and he doesn't remember his face well. Yukito also said that "It's been a while" when he was called by his first name. So I think Yukito just didn't realize it at first but remembered Imasaki later and why he was sure that he can see people's futures.
But that kinda contradict his good luck no? Cuz he went missing
That kid probably died. I think the chances of him seeing someone's death is slim so it struck him. But you know, if you are meant to die, that's meant to happen. He couldn't have prevented that for all he can.
He also kept on checking that kid's future so it meant to traumatise him either way.
what actually happened to the spirited away friend though? we never really found out how or why the kid went missing, i was hoping they'd explain that...