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Amend the soil around the plant in the fall with 1/2 lb of colloidal sulphate or bone meal, both of which are good sources of phosphorus. You may also want to add in some rotted manure if you have it. Keep in mind, however, that depending on other factors and the age of the plant, blooming isn't guaranteed but this should help alleviate the problem.

Yes, it loses its leaves in the winter.

Most wisteria varieties are hardy in Zones 5-9 and many will thrive just fine with additional protection in Zone 4.

I would have to say yes. Here is some growing info that sould help: https://www.gardeningknowhow.com/ornamental/vines/wisteria/growing-wisteria-proper-wisteria-vine-care.htm

You can damage the roots, but most wisteria are so hardy that they can take the damage. In fact, you may even see an improvement on blooming afterwards.

Most likely it is crown gall, which is common in these plants. This article will help determine if this is the issue: https://www.gardeningknowhow.com/ornamental/vines/wisteria/wisteria-diseases.htm