Plants found for sale now in bloom will be greenhouse or florist's hydrangeas. And they are often (even typically) sold without identifying names. It is very difficult to assign a specific cultivar name to them as the greenhouse forcing process often involves complicated procedures to modify flower color and bloom size - what you see now is not necessarily what you will get planted in the ground. Most macrophyllas are root hardy to zone 5 although they will be damaged by winter cold unless they are protected. Often, greenhouse hydrangeas do not show the same degree of hardiness as those grown naturally for the landscape. Growing it in a container may be a good alternative, as it can be more easily moved indoors for winter protection. Hydrangeas do not make good houseplants - ones sold like this are generally intended as temporary flowering plants. If you do grow it on in a container, it will need to be hardened off or acclimatized to the outdoors before you can move it to a porch or other location. And winter protection can be as simple as just moving the container to an unheated basement or insulated garage -- somewhere cool (some freezing OK) so the plant can go through a normal period of dormancy. Bringing it into your living room or elsewhere under normal comfortable living conditions for the winter will alter the natural process and can create problems.

I get that sometimes. It's usually from a bug that is inside a stem. Look above where you find the waxy white substance and check out the tips of stems (usually old stems). Are there any that are open/hollow (the stem looks like a straw)? If so, cut a bit below it until you get to a place that the stem is no longer hollow. Take the piece you cut off and slice it open sideways and you will usually find a little insect (larvae stage or tiny winged stage). I remember Martha Stewart always said that when she pruned her hydrangeas, she put elmers glue on the cut tips to discourage borers......I have no idea if this works.

You might get an answer if you post this in the Northwest region forum. You have stuff in your area most of the North does not have.

Well-established hydrangeas of any type can be cut back hard and they will recover. The only issue with old wood bloomers is that this sort of severe practice may well eliminate any flowering for a season or two. I have some design clients that had a series of large mopheads that were not well placed and grew well beyond the space available. So before digging and relocating to a better area, they cut back these 8-10' monsters to just around 6" in late fall. They are all currently sprouting healthy new growth. FWIW, bigleaf hydrangeas generally only have flowers in a mophead (rounded) shape or as a flattened lacecap. Domed shaped flowers are usually found on the paniculatas and the oakleafs.