The pacers have to obey the same rules as every other runner. They start at the starting line, they can't run inside the lines, etc.

One of the reasons that Eliud Kipchoge's "Breaking 2" marathon wasn't a legal world record was that teams of pacers would rotate in, take a break, then rotate in again. This way he could have pacers with him almost the entire way. But in a real world record attempt, the winner is always running by themselves in the last part of the race because all of the pacers have been left behind or dropped out -- by definition of a world record, they can't keep up to the end.

No, there is no official record with and without pacers. And arguably, that's why the Olympics is a bit rarer for middle- and long-distance world records (because you aren't allowed official pacers). You could probably track it down though.

I don't know, but define a pacer ?! Someone with better ability to pace than yourself? Someone of lesser endurance willing to run a half race at a pace they can't sustain, but you can? What's the difference between having a pacer and another world class competitor trying to beat you that brings out your best ?

What about slip streaming? Should we regard records differently where the winner stayed with the pack until a final sprint as opposed to front running the whole race?