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These articles should help:
https://www.gardeningknowhow.com/plant-problems/environmental/frost-how-to-protect-your-plants.htm
https://www.gardeningknowhow.com/plant-problems/environmental/preventing-frost-heave-in-your-garden.htm
https://www.gardeningknowhow.com/plant-problems/environmental/protect-plants-in-freeze.htm
https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm

This really depends on the vegetable, as some have should be used rather quickly while others can be frozen. The normal way to freeze vegetables from the garden is to chop them up and blanch them, spread them out on a cookie sheet and then freeze them. Once they are frozen, you can put them in storage bags and keep them in the freezer until you use them. For many others, like cucumbers, you can store them for a week or so in the refrigerator but if kept too long, they will no longer be any good. If you find yourself with an abundance of veggies, you can always give them away to family, friends, and neighbors.

No, in your area you will still have time to grow. I think you will find this guide from UG helpful for planning and planting your garden in your area:
https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetable-gardening-for-beginners.htm

Treated lumber is no longer treated with carcinogenic chemicals, so are safe to use in your vegetable garden. Some people do still believe that because they are treated with chemicals, they should not be used. Whether you do or not, is up to you, but according to science, they are safe.

This depends. What type of vegetables are you seeing this in? Some varieties are naturally purple. If this is not the case for you, then it's possible that the purplish hue is an indication of some type of nutrient deficiency, but the food itself should still be ok to eat.

This really depends on the type of chemical pest control used. Some types remain in the soil longer than others. Residual types usually evaporate after so long, making them safer. Read the directions and warnings on the label of any chemical product you buy. The manufacturer should provide detailed instructions on how to apply and when it will be safe to grow plants in that area again. If you cannot find any answers, put off gardening in that area for at least a year and try another location. You could also consider growing veggies in containers instead.