The clearance is tight for 18" especially over the rear parking brake. I don't recall coming across any talk of your specific wheels here, or even Rotiform much in general.
Since you have the car and wheels both already, I think you're in a better place than any of us to figure out how to make them fit, if possible. If the barrel clearance is greater inside the wheel then using a spacer makes sense to me. How deep of a spacer do you need? I'm no expert on spacers, have never used them, but others here can give guidance on spacer fitment. I do know that there's only limited extra stud length, so if you need more than a few mm you will need either extended studs or bolt-on spacers, depending on exactly how wide.
If you only need spacers for the rear wheels that seems ideal. You can push the rears out wider without rubbing than the fronts. The rear fenders are wider, and of course the rear wheels don't turn.