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This article will help you in correctly pruning Hydrangea macrophylla: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm

You cannot change the color of white hydrangeas to pink or blue, although some begin with white blooms, then age to a pink or tan color.

When they are not opening fully, then this is usually a sign that there is a lack of potassium and phosphorus in the soil. You can, either, amend the soil with a topdress of compost, or add something higher in phosphorus and potassium, such as potassium phosphate.

This article help you with the care of hydrangeas in the meantime: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm

The best ways to dry water saturated potting soil is of course protect the pot from additional rain or irrigation. Move it under cover or put up an umbrella.
If there is a saucer under the pot, dump out or spoon out excess water that drains through.
Exposure to sunlight and wind, even a fan, will help in the drying of the soil.
Monitor soil moisture content with your fingers or a trowel or better yet a soil moisture meter.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm

Yes, that's the advice. Since there are no roots, even in a high humidity environment, it is difficult to maintain the leaves' moisture. If they wilt they become prone to fungal infection. But the cutting needs energy from photosynthesis so some leaf surface is necessary. It is a balancing act.

This is a large mophead variety and can become heavy and hang; wind and rain can cause this to occur; not necessarily a week plant.

Water hydrangeas at the bottom of the shrub rather than from overhead. Overhead watering can weigh down the flowers, causing them to flop over.

Deadhead fading blooms, clipping them off just under the flower heads. Removing the faded blooms reduces the top-heaviness of the shrub. You can also cut off some of the heaviest flowers while that are in bloom and arrange them in a vase to enjoy them indoors.
Leave 18 to 24 inches of stems behind when you prune your hydrangea. Do this rather than cutting branches back to the ground. The thick, older stems provide a sturdy base for new growth, reducing the likelihood that the stems will flop over when in bloom.