Who approaches Dimmesdale when he invites Pearl and Hester to the scaffold?Why does this person wish to prevent Dimmesdale from mounting the...

I think that you are talking about the scaffold episode
that happens in Chapter 23.  If that is correct, then the person who approaches
Dimmesdale when he tries to mount the scaffold is Roger Chillingworth.  He tries to
prevent Dimmesdale from doing that and, more importantly, from acknowledging that Pearl
is his child and that he was the one with whom Hester committed
adultery.

In my opinion, Chillingworth does this because he
wants to maintain his hold over Dimmesdale.  If Dimmesdale confesses in front of
everyone, Chillingworth will no longer have anything to hold over his head.  He will
have no way of torturing Dimmesdale.  He wants to keep doing that and so he tries to
prevent Dimmesdale from confessing.