1) 215/35/18, and 205/40/18, are not "ALMOST" Identical. In fact, they're pretty far off:
215/40/18 = 86MM Sidewall
205/40/18 = 82MM Sidewall
215/35/18 = 75.25MM Sidewall
You will have 7MM+ of Clearance on the "Top" and the "Bottom" of the tire, coming to an effective 14MM, which is about 1/2" of Clearance. That's HUGE for somebody who's rubbing, just FYI.
2) I've been driving with:
215/35/18 on 18x8 ET32
215/35/18 on 18x9 ET30
(Both on Fronts)
I just put it on the 18x9s, and they ride completely fine. Yes, a 215/40/18 will be a bit of a better ride. However, it's not that much worse to go from a 215/35/18.
I have a friend who rides with 215/40/18 on 18x8/18x9, and when he sits in my car, when I used the 215/35/18s all around, he said it felt very similar, and just a little harsher.
3) Will you curb/bend/crack a rim?
If you're not an idiot, and know how to drive and not drive straight into a fucking hole, and take curbs at stupid idiot angles, then no. I drove on the 215/35/18s for quite a while, and only knicked my rim ONCE, but I was going in a curb at the wrong angle, completely my fault.
Learn to drive, and you'll be 100% OK.
Pay attention to the road, and you'll be 100% OK.
Don't do the above two? Go kill yourself.