powdery mildew is very common during wet weather and should not be a threat to the life of your plant. isn't it almost time for a killing frost where you live?
hydrangeas do not like water-logged soil so, if the planting area is going to become that, you might consider moving it to higher ground.
as for winter protection, perhaps someone closer to your geographic area can advise about that or search this forum using that term.

wish we could have some of your rain surplus down my way!
it is very dry here.

Does the soil drain well? It can stay wet with rain if not heavy/soggy or puddling without affecting your hydrangea....I think we've had about 3 dry days this month and before too many more weeks pass, I won't dare try to dig without destroying my soil structure until winter wet is over. No harm to my hydrangeas.

As for the PM, I don't think I'd worry about it this late in the year. Just pick up leaves that were infected when doing your final Fall cleanup so the PM spores don't winter over at the base of your shrub.