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You can do the formula but the overall outside tire diameter is suppose to remain the same within a certain percetage so theoretically by going from a 17" to an 18" you will lose 1" of total sidewall. This translates to 1/2" each side so 2 3/4 - 1/2 = 2 1/4" worth of sidewall.
If you want the more precise formula go to Tire Rack's site and it will provide a detailed explanation on how to use the tire's size marking to translate into width and sidewall.
I don't think the sidewall is any stiffer on an 18" than a 17" tire of the same brand and model. You might feel an increase in stiffness but it would be because you are taking the 1" diameter difference and changing from rubber to metal.
Sorry I can't help with the wheel questions.
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