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Water alone is sufficient in washing them. You can also soak the vegetables in water for a few hours prior to giving them a thorough rinsing off. The normal way to freeze vegetables from the garden is to blanch them. Drop the Brussels sprouts in boiling water for about three minutes and then transfer them to ice water. Pat the veggies dry and place them in storage bags and freeze until you use them.

Brussel sprouts might be tough to start with, but that being said, go ahead and try. The fun of gardening is that you can try things like that. Many people start with tomatoes, carrots, squash and radishes as these are some of the easiest to grow.

Here is an article with some other suggestions: https://www.gardeningknowhow.com/edible/vegetables/vgen/starting-a-vegetable-garden.htm

Brussels sprouts would have a hard time upside down and a large coffee can would not provide enough soil for their root system. I would recommend that you aim more for a 5 gallon bucket for this plant. Many hardeware store sell them cheaply and many restaurants may give then to you free if you ask.

This article should help with that: https://www.gardeningknowhow.com/edible/vegetables/brussels-sprouts/harvesting-brussel-sprouts.htm

Neem oil is effective against many types of insect pests and is safe to use. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

It has powdery mildew. I like neem oil or you can use copper sulfate for its treatment. Here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm

Also, for the aphids, the neem oil will take care of them too and is safe to use. The Brussels sprouts will still be edible as well. Here is more info on neem oil: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm