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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.

This article will help you with that:
http://www.no-dig-vegetablegarden.com/straw-bale-gardening.html

There are many reasons for leaf yellowing in vegetables plants. Here is information on the most commonly grown: https://www.gardeningknowhow.com/edible/vegetables/tomato/yellow-tomato-leaves.htm
https://www.gardeningknowhow.com/edible/vegetables/pepper/pepper-yellow-leaves.htm
https://www.gardeningknowhow.com/edible/vegetables/squash/why-squash-leaves-turn-yellow.htm

There is. It is usually made with Epsom salts, but studies have shown that foliar application of calcium is not very effective in fixing blossom end rot. It has some effect, just not as helpful as some people believe.

Once the flowers start to dry, you can cut the flower heads off and place them in a paper bag for a week or two to finish drying. Then shake the bag to loosen the seeds. Most of the time, you need to replant to get them to grow again.