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I would guess that the nitrogen to phosphorus ratio is off. The soil may have an abundance of nitrogen, which would result in nice, healthy green growth but little to no blooms. Without the right amount of phosphorus, it will not bloom well. Bone meal is a great way to add phosphorus. This article will also help: https://www.gardeningknowhow.com/ornamental/vines/wisteria/wisteria-blooming-problems.htm

If the trunk is damaged, I would not purchase it. While it can recover from this kind of damage, it does increase the chances for disease, which could kill the plant or at the very least damage its ability to grow and flower. Look for healthy specimens.

Unfortunately, there is no way to change the color of the wisteria. If it is a variety that is light purple, it will stay light purple and you will need to plant a new, darker flowered variety to get the color you like.

I would cut it back to where the stems are still alive. This article will help you figure out where that is: https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm

Did you have a late frost in your area? If so, it likely killed the blooms. The leaves are less frost tender than the flowers, so it is not uncommon for a frost to wipe out the blooms and leave the rest of the plant undamaged.

Yes, remove all the flowers. Soilless mix is a soil that has no dirt in it. Standard potting soil typically meets this description, so you can use plain old potting soil. Rooting hormone should be available in most garden centers. It comes in a little jar.