Fertilizer won't affect the butterflies, especially if you use a pelleted time-release type. Just sprinkle some on the soil around the plants, then water as usual so it will release over 3 to 6 months (depending what kind you get).
I don't bother fertilizing milkweed, as the monarch caterpillars will eat it before it gets to have any nutrient deficiency problems anyway. Expect to have just sticks pretty quickly once the caterpillars get to feeding on it.
As far as bugs go, you're right, you don't want to spray with anything toxic, but a good misting with the hose once in a while will wash off a lot of mites and aphids but won't hurt your butterflies. Check first for baby caterpillars though, as they can be washed off along with the bugs.
