I really need to get to my evap canister so I can get rid of the permanent CEL but all the bolts seem rusted and seized.

 

probably more for looks and a cover to help keep weather away from canister and lines.

 

Yes, those small 10mm bolts are very easy to snap. If you do get any out, I would put anti-seize on the threads to make it easier later.
I do this for almost every bolt I remove.

I suspect the rear cover helps to direct air under the car and around the bumper. With out it, a large cavity is exposed which might "catch" the air flow, but I don't think you would notice any difference.