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Make sure that you know that this is an actual boron deficiency before you apply anything like this, as it is rare for there to be a deficiency like this. Adding more than is necessary will do much more harm then good for plants, as it is very easy to create a toxic environment with this.

If you can include photos, I can help you to determine the best course of action for the issue at hand.

Sounds like Powdery Mildew.

Here is a link to help you.
https://www.gardeningknowhow.com/edible/vegetables/squash/powdery-mildew-in-squash.htm

What are you growing them in? They need loose soil like potting mix to send down roots. More root growth then spurs top growth. If you are growing them in garden soil, 100% compost or straight coir/peat then I would add perlite to the mix and repot them. If your containers do not have drainage holes in the bottom, root rot is a problem that will limit top growth. Good drainage and letting the soil dry out between waterings is important. Please post a picture if you have problems despite good growing conditions.

This is likely due to the lack of humidity. If you can spray them early in the morning, then this can help some. You may also want to add dolomitic lime to keep the pH stable.

This article will give you more information on the care of these plants: https://www.gardeningknowhow.com/edible/vegetables/zucchini/growing-zucchini.htm

Yes, and you can keep this pollen for longer in the refrigerator. I have to do this all the time, as I usually only do one plant at a time indoors. You can either save the whole flower, or just clip the stamen out and keep that.

Do you have bees around the flowers. If no pollinators are visiting, you need to pollinate the flowers yourself. Here is info on hand pollinating melons then zucchini: https://www.gardeningknowhow.com/edible/fruits/melons/hand-pollinating-melons.htm
https://www.gardeningknowhow.com/edible/vegetables/squash/pollinate-squash-by-hand.htm