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Yes, I would treat it like an ornamental grass and trim it back each year. Do it now, this year, but in the future, do the trimming in either late fall or early spring.

Here is an article that you may find helpful: http://ucanr.edu/sites/scmg/Sonoma_Gardener_Articles/Gardening_on_a_Hillside/

For additional ideas, try these articles as well: https://www.gardeningknowhow.com/special/slope-hill/watering-hillside-garden.htm
https://www.gardeningknowhow.com/special/slope-hill/growing-vegtable-garden-hill.htm

Just cut it off. To reduce the size of the clump, wait for decent weather (you don't say where you live) and divide the clump and then replant what you want.

Lemongrass prefers to be grown in sandy-like soil that is kept evenly moist. Turn over the soil well so that it is well drained. You do not have to add fertilizer to the soil before planting lemongras, but an application of mulch will help retain the soil moisture. This article has more infromation on growing lemongrass: https://www.gardeningknowhow.com/edible/herbs/lemongrass/learn-about-growing-lemon-grass-plant.htm

It depends on how it is cared for. Most clumping herbs, like lemon grass can live essentially forever if they get the proper care and are divided every few years to reinvigorate them. If you find that your lemon grass is dying out, especially from the center out, then it may be time to divide it. The dividing forces the plant to rejuvenate and grow itself, and bring it back to its previous vigor. Plus, you now have plants to share with friends as well.

Yes, you can cut them back. They respond well to being trimmed.

Indoors, they will need high light, as much light as you can give it. Water only when the top of the soil is dry to the touch. Even then, you should expect that keeping it over winter is more of a way to make it survive, rather than thrive. It will begin to look rather ragged by spring, but will bounce back quickly when placed back outside.

For this reason, many people choose to put the plant in dormancy for the winter. This is simply putting it in a cool, dark, dry place for the winter and watering once a month until you can place it outside for the spring and resume normal care.