I think it was the head that made Scrooge want to change his attitude because Scrooge didn't want to end up like that and wanted to get his act together.

Scrooge decided to change his future by fixing his act

The ghost pointed to the head to show Scrooge how he would end up if he didn't change his act.

He thought it was the last of the spirits.

Possibly so he doesn't end up like Marley.

So he can show people how grateful he is for living.

It was Scrooge's nephew.

He could not bare to see what was happening and demanded the ghost to take him home.

The ghost said he was The Ghost of Christmas Past.

He wished for happiness and pleasure.