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Trimming off spent blooms is normally done in mid-summer. While you do not have to cut it back in fall, if you would like to do so for appearances, this is fine and should not harm the plant. However, in cooler regions it is better to leave the old growth over winter for added protection and simply cut it all back prior to new growth in spring.

This plant doesn't require any special care over winter as it will die back and regrow in spring. If it looks ragged, however, you can trim it back some.

Yes, they can be grown in containers. Just care for them as you would those in the garden, though water more often when temps are hot. This article will help with its general care: https://www.gardeningknowhow.com/ornamental/foliage/artemisia/silver-mound-care.htm

We usually divide ours in the fall so it can be re-establishing itself over the winter. This article should help: https://www.gardeningknowhow.com/ornamental/foliage/artemisia/silver-mound-care.htm

While disease or insects could be at work, it's more likely that your plant just needs to be divided. Read more: https://www.gardeningknowhow.com/ornamental/foliage/artemisia/silver-mound-care.htm
You might be interested in knowing about the Agricultural Extension Service, which is paid for by your taxes, and exists in almost all counties in the USA. They have wonderful resources for helping and educating home gardeners. You can locate the nearest one with this link:
https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm

You will want to prune your silver mound in the summer after it blooms. I would recommend initially pruning back by up to a third of its height and spread.

For more information on the care of silver mound, please visit the following link:
https://www.gardeningknowhow.com/ornamental/foliage/artemisia/silver-mound-care.htm