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Unless you already know about your soil's health, I would have it tested. Fertilizing corn should be done on the basis of soil tests. Every type of soil is different and has a different chemical make up that requires different fertilizers. That being said, it is usually better to use manure or compost whenever possible; but in addition, fertilize with a commercial fertilizer such as 10-10-10. Corn should be lightly fertilized prior to planting. It should be fertilized again when the plants are approximately 4 inches tall and again when they are 8 to 10 inches tall. Approximately 1/4 pound of complete fertilizer for every 10 feet of garden row is sufficient in most areas. If you are not using manure, you may also want to consider side dressing with a nitrogen fertilizer 3 inches from plants once they reach about a foot tall.

I'm not sure of the zone you are gardening in, but it could be a few different critters.
Mice, Voles, Squirrels, Chipmunks to name the most common.
I have used chicken wire tunnels over freshly planted seeds with some success in my own garden.
You can cut strips of chicken wire and make a half circle cage over the freshly planted seeds. Use landscape anchors to pin the wire into the soil. It may help to deter the pest.

https://www.gardeningknowhow.com/plant-problems/pests/animals/camouflage-gardening.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/blood-meal-fertilizer.htm

Here is a link that may help you.

https://entomology.ca.uky.edu/ef318

In the husk, in the fridge -- but keeping it in the husk is a matter of preference. You can shuck it and then store it, too.

I keep mine in a plastic grocery bag so that a little air can still circulate around it, otherwise you'll get a slimy feel to the husk when you take the ears out of the fridge (if it doesn't get eaten quickly).

Here's an article that may help you:

https://www.gardeningknowhow.com/edible/vegetables/corn/tips-for-harvesting-corn.htm

It could be bad seed like you suggest. Or, this could be a case of cross-pollination of your sweet corn with a non-sweet variety growing nearby. If there are farmers nearby growing other varieties, pollen from those varieties could have drifted over and caused most of your kernels to be less sweet than they should.

https://www.gardeningknowhow.com/edible/vegetables/vgen/cross-pollination.htm
https://www.gardeningknowhow.com/edible/vegetables/corn/corn-pollination-how-to-hand-pollinate-corn.htm

This article talks about mingling of GMO and non-GMO corn pollen, but this can also happen with any variety:

https://www.agry.purdue.edu/ext/corn/news/articles.00/gmo_issues-000307.html

This looks like corn smut, also known as Huitlacoche. It is a fungal disease, but infected ears are also eaten as a delicacy in Mexican cuisine. If you want to eat this corn you should probably check with a local expert to make sure it really is corn smut and safe to eat. Otherwise, there are ways to prevent its occurrence. See the following articles for more information:

https://www.gardeningknowhow.com/edible/vegetables/corn/corn-smut-fungus.htm

https://blog.mycology.cornell.edu/2007/11/13/huitlacoche/
http://ipm.illinois.edu/diseases/series200/rpd203/