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Give Square Foot Gardening: A New Way to Garden in Less Space with Less Work by Mel Bartholomew a read. The author explains what to start planting and when and how to efficiently organize your garden into square foot areas, including their size. Another good read is Lasagna Gardening by Patricia Lanza. This book is one of my favorites. It’s extremely detailed and informative, packed full of useful tips, and written in easy-to-understand terms for the beginner. Both books will be invaluable resources for your square foot vegetable garden.

I don't think it is bolting. How are you watering it? From above or below? If you are doing it from above, I would try watering from below instead. Also, don't let the soil dry out completely before watering. It should just be dry to the touch on top. If you stick your finger into the soil, it should still be damp. This is the correct time to water it.

As long as the vermiculite was purchased recently, as in the past few years, there is no concern about asbestos. The asbestos was found in vermiculite harvested several years ago and would not be in today's supply.

Rule of thumb is for vegetables where you eat the leaves, you want something that has a slightly higher nitrogen (the N in NPK) and for vegetables where you eat the fruit, you want something with a slightly higher phosphorous (the P in NPK).

If you are harvesting with the intention of harvesting lettuce from the plant again, I would leave 1-2" at the base on the plant.

If moles or squirrels are a problem, these articles should help: https://www.gardeningknowhow.com/plant-problems/pests/mole-control-home-remedies-for-removing-moles-from-your-yard.htm
https://www.gardeningknowhow.com/plant-problems/pests/animals/get-rid-squirrels.htm

It could also be slugs. This article will help determine if slugs are a problem, and if so, what to do: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm