KatyFernLily already provided a link how to edit premade worlds in the other forum you asked about this. As for your second question, most worlds will load even if you don't have all required EPs installed. There might be severe problems though. For one, not all missing objects are replaced by Basegame objects, some will simply be missing. It doesn't matter much with decorative objects. But missing doors/arches/stairs/elevators will cause routing problems.
Even worse, as you don't have IP, worlds relying on boat routing won't work at all for you. Same with worlds that use bridges from an EP you don't have installed. Those will simply be missing and thus block important routes.
