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If you do not fertilize these each year then this will be the issue. This article will help: https://www.gardeningknowhow.com/edible/fruits/grapes/growing-grapevines.htm

This article will help you.

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

We have just the article for you! This article will help you to propagate grapevine cuttings: https://www.gardeningknowhow.com/edible/fruits/grapes/rooting-grapevines-tips.htm

Grapes will be suited to grow in container, actually. It will need a fairly large container. At least 15 gallons. The problem with planting in your zone isn't necessarily the heat, but that it won't get enough chill hours in the winter. Especially Concords, since they are marketed as a cold hardy variety. There will be nothing that you can do to combat this. It may still grow and produce a little, but I would not expect a good harvest from these in your area.

This article will give you information on the care of grapes in container: https://www.gardeningknowhow.com/edible/fruits/grapes/container-grown-grapes.htm

My best guess would be climate related. There are many microclimates within each area, and sometimes if it is just a little colder or warmer than usual, then you may see a disruption in production. It also could be related to lack of pollinators. I would be willing to bet that they will return to production by next year. This collection of articles will give you more information about the care of grapes in the meantime: https://www.gardeningknowhow.com/edible/fruits/grapes

Yes, this is common practice. They are very nutritious, and packed full of beneficial chemicals! I think that you have a very good idea.