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Difficult to say from description.
Leaves can wilt from over or underwatering, leaf curl can be due to pests or disease.
Wind damage can also cause leaves to die off.
https://www.gardeningknowhow.com/ornamental/vines/hyacinth-bean/purple-hyacinth-bean-care.htm

The purple Hyacinthe is, indeed toxic. Most of the toxins are within the beans though. Cattle are a little less prone to being injured by the toxins, however, You should still take precaution. These beans aren't necessarily deadly, but they will cause major upset to the stomach. Please remove these from the area that the cattle can access them. Now as a side note... You can prepare these beans in such a way, as to make them edible to you. The important thing is to pick beans before they ripen into viable seeds. DO NOT CONSUME BEANS RAW! These require boiling, then turning off the heat, letting them soak for an hour. Discard the water, repeat at least three times. after the third time, cook them as you would green beans. Now, if you must consume ripe beans, make sure to follow the above process but give them an extra soaking to ensure you have removed the toxins. The flavor is unique, and the nutrition profile is quite impressive. cook as you would black beans, or pintos.

Yellowing leaves could be watering issues; too much or too little, pests, disease or environmental issues.

Pests can be treated with Neem Oil, which also works as a fungicide.
Hyacinth Bean plants need lots of water, but soil should never be soggy.

The articles below will refresh you on the care requirements.

https://www.gardeningknowhow.com/ornamental/vines/hyacinth-bean/purple-hyacinth-bean-care.htm
https://www.gardeningknowhow.com/ornamental/vines/hyacinth-bean/hyacinth-beans-in-pots.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Yes and if you have the means to till the foliage into the ground, then that will give your soil some extra nitrogen for growth next season. Here is an article with more information: https://www.gardeningknowhow.com/ornamental/vines/hyacinth-bean/purple-hyacinth-bean-care.htm

Hyacinth bean plant is not winter-hardy and can be killed by any frost, so it is typically grown as an annual in temperate regions. You can wait until the weather kills them or pull up the plants when they're no longer attractive.
Lack of sun can certainly cause poor flowering. Here are some other possibilities:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-not-blooming.htm

We don't usually recommend places to buy seeds, but I would check online. There are many nurseries that will ship just about anywhere.