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Yes, if you get flowers on any herb, it is best to take the flowers off. The flowers sap energy from the plant and it produces fewer and less flavorful leaves.

I would start pinching the plants at a rather early stage. This will encourage branching. This article has more information on that:
https://www.gardeningknowhow.com/edible/herbs/hgen/making-herbs-bigger-through-pinching-and-harvesting.htm

Some varieties grow back better than others. I would recommend purchasing a named variety from a reputable nursery. While you keep the ground dry, snow and rain in the winter can be over watering it in the cold, which will kill the roots. A more hardy variety will be better able to survive this.

This happens to herbs when you are too nice to them. The better their conditions, the weaker and more diluted their flavor becomes. They need to struggle a bit to have good flavor.

At the very least, do not fertilize your tarragon. Do not provide more than 2-3 inches of water a week, if you can help it.

Do not harvest all of the leaves, only what you need. Also, pinching the plant will promote more bushier and lush growth. Here is more information: https://www.gardeningknowhow.com/edible/herbs/tarragon/growing-tarragon-in-the-herb-garden.htm
https://www.gardeningknowhow.com/edible/herbs/hgen/making-herbs-bigger-through-pinching-and-harvesting.htm

Well, depending on the type of tarragon you have, I would guess about 90 -120 days after planting (seeds) that it would be mature. They also grow 2-3 feet tall.

In your region, they should not have too much trouble coming back, especially with a generous layer of mulch in winter. You can, however, grow this plant indoors as well. You don't water plants over winter, though those grown indoors may need watering once a month just to keep the pot from becoming overly dry. Placing the plant in the shade during hot weather is a good thing, though remember that potted plants require more watering than those planted in the ground (daily, sometimes twice, in temps over 85 F). These plants share similar growing requirements as regular tarragon, so these articles should help: https://www.gardeningknowhow.com/edible/herbs/tarragon/growing-tarragon-in-the-herb-garden.htm, https://www.gardeningknowhow.com/edible/herbs/tarragon/growing-tarragon-indoors.htm, https://www.gardeningknowhow.com/edible/herbs/tarragon/growing-french-tarragon.htm Also, If you are trying to cover a large area with these plants, you will need more than one. To get you plant bushier, you may need to pinch it. This article will help with that: https://www.gardeningknowhow.com/edible/herbs/hgen/making-herbs-bigger-through-pinching-and-harvesting.htm