Hi Danny23
The van looks to have had some use ! You might need to get a body shop to straighten out some of the biffs before you start fine tuning.
On the leading edge of the door (Photo 2) there should be a conical head, where there is presently a hole in the short strap piece. This engages with a plastic cup insert in the diecast piece shown in Photo 4. There should be upper and lower cone and cup pairs which set and hold the alignment of the front of the door when it's fully closed. Try Coastal Motorhomes for replacements. I found that marking the cones with chalk allows you to see where they touch the sides of the cups, so that you can adjust them in the slot holes for an exact mating.
The door weight is taken by the upper rollers, with guides at the halfway point (stainless channel on the van side) and at the bottom. The guides are just that, they don't take any appreciable weight. The various rollers should be greased in their bearings so that they turn freely, and should be round and unworn. Again, replacements are available and bolt-on. I found it best to leave the tracks ungreased, as grease picks up dirt and makes the rollers skid instead of turn which is not what's needed.
There are various places where the roller brackets can be adjusted, but without seeing your van in person I can't say exactly what needs tweaking. You should be able to work out what does what. The aim is to get a door that slides easily and shuts with reasonably even compression of the rubber seal all round. Don't worry too much about even shutlines, it's a van not a BMW !
The screw looks like a bodge attempt at an end stop. The real end stop and releasable holding catch (connected by bowden cable to the door handle) is at the lower trailing edge.
