Could cystic acne infections cause slightly elevated white blood cell count?

Yes. Or simply stress -- even a cold shower can raise it to 20, 000. I'm seriously concerned that you describe "cystic acne" but only list benzoyl peroxide. That's not nearly enough. You need to be under the care of a competent physician, probably a dermatologist, as cystic acne usually requires Isotretinoin or something else really powerful. Best wishes.