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These articles should help answer your questions: https://www.gardeningknowhow.com/edible/fruits/grapes/rooting-grapevines-tips.htm

https://www.gardeningknowhow.com/edible/fruits/grapes/prune-grape-vine.htm

I use a product called Green Cure to keep things like powdery mildew, rust and black spot off my roses, pumpkins, cucumbers and such. I see no reason why it would not work very well on your grape vines. Here is a link for you on this product so you can check it out;
http://www.greencure.net/

This article should help: https://www.gardeningknowhow.com/edible/fruits/grapes/growing-grapevines.htm

Here are 3 links to articles with information and diagrams showing how to prune grape vines:
https://www.gardeningknowhow.com/edible/fruits/grapes/prune-grape-vine.htm
http://wine.wsu.edu/research-extension/files/2013/02/GrapePruningBasics.pdf

There are several sources of the seedlessness trait, and essentially all commercial cultivators get it from one of three sources: Thompson Seedless, Russian Seedless, and Black Monukka, all being cultivars of Vitis vinifera. There are currently more than a dozen varieties of seedless grapes. Several, such as Einset Seedless, Benjamin Gunnels's Prime seedless grapes, Reliance and Venus, have been specifically cultivated for hardiness and quality in the relatively cold climates of northeastern United States and southern Ontario.
I did not find any referance to a seeded Thompson Grape, so I assume that all Thompson Grapes are seedless.

There is probably a nutrient imbalance in the soil - likely too much nitrogen, which produces lush growth, and too little phosphorus, which is responsible for flowering and fruiting. I would recommend adding some phosphorus rich fertilizer or bone meal to the soil. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm