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It is not an edible plant, really. The flowers used to be eaten, but the leaves are definitely not edible.

Yes, spring would be the ideal time to trim the plant back, especially any dead or unhealthy growth.

Before resorting to chemicals, try hand pulling as much as possible. Then you can pour boiling water on the remaining plants that are becoming 'weedy'. Be aware though that this will kill any plant the hot water also comes in contact with. If you still want to give Roundup a shot, simply use a shield of some kind (cardboard works well) to protect the main plant and spray the weed killer on the plants you want to get rid of.

Just clip off the unwanted plants.

Using a herbicide will kill the main plant, likely and the risk of accidental exposure to other plants is high.

https://www.gardeningknowhow.com/ornamental/flowers/russian-sage-flowers/russian-sage-care.htm

You can take a root cutting at really any time but if you would like to start with cuttings, do so in the spring.

Here is a link with more information.

https://www.gardeningknowhow.com/ornamental/flowers/russian-sage-flowers/russian-sage-care.htm

The easiest and most common way to propagate Russian Sage is through division or cuttings in the spring.
You can however collect seeds.
When the flower sepals turn brown and dry, the tiny seeds are about 1/32 " in diameter.
You do not have to separate the seeds from the chaff but you will need to give the seeds a cold period.
This is called stratification.

Here are some links to help you.
https://www.gardeningknowhow.com/garden-how-to/seeds/seed-stratification.htm

https://www.gardeningknowhow.com/ornamental/flowers/russian-sage-flowers/russian-sage-care.htm