A comment about offset first. If you were able to look down through the wheel from the top, if the mounting surface of the wheel plate is dead centre (inside to outside) you have zero offset. If the plate is closer to the outside of the wheel, which pushes the wheel inwards toward the body, that's positive offset. If the plate is nearer the inside of the wheel, pushing the wheel out, you have negative offset.
You are thinking about getting clearance by moving your wheel out further, which means LESS offset e.g reducing from 42mm to 35mm.
Your steelies probably have a closed/solid front face so you can't look through them, and getting a look from under the car is a mission, but a look is what you need, to see exactly what the problem is.
I have an open spoke design on my alternate summer tyre rims, so I'm afraid I have to tell you that the calipers, front and rear, are completely inside the wheels, so if you have rubbing on a caliper, any amount of offset change won't remedy that UNLESS the wheel inside diameter where the calipers sit increases a little toward the suspension side of the wheel. So if the inside face of the wheel is dead flat and parallel with the outside face (where the tyre sits), you are out of luck.
It is possible, if unlikely, that the problem instead is that the wheel is too close to the bodywork - you would have to eyeball that to tell. A grinding noise suggests not...
You did all the right checks first, except that going with a smaller wheel than stock cancels that unless you take the car in to a tyre shop and have them confirm before purchase. The Kona rear calipers are larger than the front ones unfortunately, and a neat fit even with some 17"s...