I fully agree that this method of UV mapping is not the best way for something complex. Just to test, I made a low-poly airplane and used that method - it worked OK, but not something I’d recommend - you can see that the wings do not get enough of the UV space - but that can be fixed by sorting out the layout.
For anyone else following this, the way to use this method is to make a shading domain for each surface, hide them all, then, one at a time, reveal, do the projection mapping, validate, put the map way to one side so that the following maps don’t overlap. Hide that domain, reveal the next and repeat. Once it’s all done, reveal all then pull the maps onto the grid, resizing and laying out.
Mine only had 28 domains and this was done without any problem - possibly 40+ would be too many. BTW, this only works for projection mapping, not for unwrapping using seams.
My method for identifying parts of the UV map when texturing is to have the model and the map open in my rendering app while the 2d paint app is open. I highlight some polys on the 3d model, which show up on the map in the 3d app, then take a look at where they are in the 2d app.