ok...you have watered it too much...you have a shady spot with evening sunlight...endless summer do not wilt because of the sun in a spot like that..it will however wilt because the plant's roots being rotted out due to overwintering...do not water anymore, in fact, since this is a new plant, I would dig up, put into a pot with some potting soil straight out from a bag (miracle growth, scotts or whatever brand), put it in a sheltered spot (away from direct hot sun, wind, rain) and see if the plant lives....if you get phytothora wilts - the soil where the plant used to live is infected with the fungus as well, remove them....your veggies getten powdery mildew could be a sign of the conditions of the spot - way too wet, not enough ventilation...and you keep on watering the hydrangea = massive fungal parties there...take the hydrangea out from the soil and see if it revives...my fingers are crossed though...you may also want to add some perlite to the soil water the hydrangea is planted - improve the drainage a bit...