But why did terra need to stand up to the darkness before helping her?

To not do the same thing he just did?

Ven just walked to maleficent's Castle and touched the heart and aurora was safe.

Cause Ven didn't have to stand up to the darkness, unless you're implying Terra could have done the same thing and just chose not to.

I don't remember this one really, but didn't Terra just wanted to meet the thing he was talking about.

As for eraqus, with terra being one of his students, wouldn't you think that he would trust his master over xehanort who has been shown to not be nearly as present in his life?

It was a split moment decision, plus like Terra said, Eraqus was literally about to kill Ven, so it wasn't likely that he could talk to him. It's not so much he trusted MX over Eraqus, it's that he didn't have another decision to make in that split second, and once he started fighting, the darkness took over. Of all the three things you mentioned, this is the most ridiculous one not to understand.