Why is Doc inside the Delorean that is in the truck at the mall in the beginning?

Yes.

Obviously Einstein is secretly capable of driving and closing said van. Obviously.

A dramatic entrance. No more than that.

Why was Doc in there? How long was he in there? What was the point?

If you want a logical in-story explanation, then a good one might be the following:

It had been earlier in the evening before Marty showed up and so Doc Brown didn't want to attract suspicion and attention by tinkering with his futuristic contraptions, and depositing plutonium into the flux-capacitor while wearing a radiation suit while patrons were leaving the Twin Pines Mall (or is it "Lone Pine", I forget) upon it's closing. Once we reach the point in the story where Marty arrived, the coast is obviously clear, and he doesn't see the need to return to the truck to refuel. Plus, Marty is filming the process for posterity.

Of course, nowadays, his experiments wouldn't fly at any hour at that location, but it was simpler times without covert CCTV everywhere in commercial districts. He just needed to wait till it was late enough.

That's really all there is to it. The function of the box truck is obviously as a mobile garage.

I suppose you could also deduce that Doc Brown was inside jerking off and imagining the possibilities regarding all the tail he's about to receive throughout history with the confidence of being the all-time greatest scientist in human history. He could probably go back and bang Cleopatra if he so desired, and live like a Pharoah for a week.

The first scenario seems more likely. :-)