Yea there will be holes in the bumper, i had in
ibiza before my leon, and when i took it off there were holes left where my splitter was (i moved it to leon) but you cant see them unless you look under the car. even then, they're only screw holes. you cant see them with the splitter on.
Screw rust... mmm i dont think there is much of a
problem they'll not rust enough for the splitter to drop off, you can always buy treated screws that won't rust i think. Like i said i used a few bolts with some washers so the load of the splitter is distributed a bit more so if i scrap it the screw won't rip through the bumper... so the theory goes. I have used a couple screws aswell. Its been like that for 3 months not a
problem so far. and my mate who fitted it has had one on his on for over a year.