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You are going to have cut the stalk back to about 2 inches, and repot it into fresh soil. Bananas will get infections, almost, out of nowhere.

You can prevent this in future by making sure there are plenty of drainage holes in the container, using a well draining mix, and making sure to only water when the soil is almost completely dry down to about 2 or 3 inches. This will prevent infections. It can be quite difficult to treat an infection once it starts, except to make the plant start fresh.

This article will give you more information on banana problems and how to treat them: https://www.gardeningknowhow.com/edible/fruits/banana/banana-pests-and-diseases.htm

Container bananas will, actually, prefer a good bit of shade throughout the day. It will often get too hot in container in full sun.

Now, the reason for this condition will likely be fungal in nature. Bananas are very susceptible to rot issues.

Since it is in container, the solution can be simple: When you water next time, make sure to replace 1/4 of the water with peroxide. You can do this every few months to keep infections away.

Bananas are heavy feeders, and will require a food with moderate nitrogen, but high phosphorus and potassium. You should apply this once a year in slow release form, or every few weeks if it is liquid.

This article will help you to care for bananas: https://www.gardeningknowhow.com/edible/fruits/banana/growing-cold-hardy-bananas.htm

This article will help you.

https://www.gardeningknowhow.com/edible/fruits/banana/indoor-banana-plant.htm

It appears to be egg clusters from an insect. Not sure which.
The insect may or may not be a pest; it could be just using the banana for reproductive habitat without feeding on the plant.
You can inspect daily and determine if the eggs hatch and what kind of larvae they are and if they begin feeding on the plant, or not.
Of if you want to take preventive action without the wait and see approach, you can wipe them off with a rag or cotton glove on your hand, or try washing them off with a hard spray of water, or smother with a horticultural "summer oil" or 70% neem oil. (The stronger concentration of oil used as "dormant oil" will burn the banana leaves.)
Here's a couple of examples: https://www.domyown.com/monterey-70-neem-oil-p-2841.html
https://www.domyown.com/monterey-horticultural-oil-rts-p-2838.html

Since you have a variety that is well suited for container, I would recommend growing these in large containers. The only way to winter these in your area will be to bring them inside.

You will want to dig these up when then start to die back, and put them in container to bring into your house. When the weather warms back up, you can put them out again. Otherwise, you will be replanting each year. There will be no way to get the roots to survive outdoors in your area. Only hardy banana species will be able to survive there.

This article will help: https://www.gardeningknowhow.com/edible/fruits/banana/banana-plants-in-winter.htm

This will depend on if you have the hardy banana or not. If this is a fruiting banana, then it will not survive. If this is a cold hardy banana, then a layer of mulch or hay, or even burlap should help during the coldest portions of winter.

This article will help you to winter a banana if you are unsure of the variety: https://www.gardeningknowhow.com/edible/fruits/banana/banana-plants-in-winter.htm