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Unfortunately, after they bolt, there is not much you can do but plant them again. This article will give you some tips for keeping them from bolting in the future (and would apply to both plants): https://www.gardeningknowhow.com/edible/vegetables/broccoli/bolting-broccoli-growing-broccoli-in-hot-weather.htm

You may find these articles helpful: https://www.gardeningknowhow.com/edible/vegetables/cauliflower/growing-cauliflower.htm
https://www.gardeningknowhow.com/edible/vegetables/cauliflower/harvest-cauliflower.htm

This article should help with that: https://www.gardeningknowhow.com/edible/vegetables/cauliflower/growing-cauliflower.htm

It may be getting a bit too warm in the cold frame you are using. Even if it is cool outside, inside a closed coldframe, it can get rather warm on a sunny day (think how your car gets warmer when it is sunny even though it is cool outside). Check the temps inside, especially on sunny days. You may need to prop it open a bit during the day to allow some of the heat out.

You will only get one head per plant, but if you cut the head and leave the root in the ground, it is possible that the plant will develop sideshoots, which can produce miniature heads.

No, cauliflower typically produces only one head per plant.