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These are perennials, and they will also tend to self-seed if they flowers are left on after they die. You may either cut the dead flowers off or leave them, but it is not necessary to cut them down.

Here is an article for more information: https://www.gardeningknowhow.com/ornamental/flowers/cardinal-flower/growing-cardinal-flowers.htm

Cardinal flower loves moist soil but does not like to be in wet soil during winter dormancy which may cause root rot. . It is also a short-lived perennial so must be encouraged to self-sow. It is gorgeous and loved by hummingbirds and hummingbird moths like the sphinx moths. To attract wildlife, make sure to plant at least 3 of them. An isolated plant won't be worth the trip; hummingbirds are looking for larger feeding areas. Or combine cardinal flower with bee balm which hummingbirds also visit. So the answer to your question depends on soil moisture levels in winter.

You aren't alone in having trouble growing cardinal flower. It must have damp to wet soil in summer but the same conditions will kill it in winter. In areas with hot summers, provide shade from the afternoon sun. Where it is happy, it will self sow; otherwise, even in optimal conditions it will only last 3-4 years. My neighbor has a lovely lupine. I can not get it to grow. Every yard has several "micro-climates" and if yours doesn't have a good match for the few plants that are this fussy, then maybe it is time to stop trying cardinal flower. As long as you meet the plant's basic light and moisture requirements, having trouble doesn't mean you aren't a good gardener. It just sounds like a poor fit for your garden conditions. Try cardinal vine; it has lovely red flowers that hummingbirds love.

It is likely to be a Pythium root rot. This infection will remain in the soil.

This article will help: https://www.gardeningknowhow.com/plant-problems/disease/root-rot-in-garden-plants.htm

This is typical. They do not last long, at all, and will continue to replace blooms as quick as they fall. Try to ensure that there is plenty of phosphorus in the soil to ensure proper blooming. You can test your soil. These articles will help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm