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Yes, you can cut your ornamental grass back now.

When cutting back ornamental grass, you should do so in early spring, as its foliage helps protect the plant throughout winter. However, it is common to have inconsistent growth with these grasses, especially fountain grass. Many of the warm-season ornamental grasses won't start growing until mid to late spring, or even early summer. If there are still no signs of life by late spring/early summer, chances are good that the plants did not survive winter and will need to be replaced.

While it is true that mice will occasionally nest in garden spots offering lots of shelter, including grasses, it is highly unlikey that they will take up residence anywhere there may be lots of traffic--such as the front porch where most people go in and out frequently. If, however, mice should become a problem for you, this article has some suggestions that should help: https://www.gardeningknowhow.com/plant-problems/pests/animals/garden-mouse-control.htm

If they are still alive, your best bet would be to dig them up and then treat the area with weed killer. To be honest, you may want to place an ad on CraigsList saying that someone can have them for free as long as they dig them up. You will likely find several people happy to do the work for you.

The berries are mildly poisonous. Fortunately, they are not life threatening if they are eaten, but do cause stomach upset and diarrhea.

It is actually better to use organic rich potting soil rather than using regular garden soil. Regular garden soil can compact and harm the roots while potting soil will stay aerated, which will give the roots room to grow well. You can easily remedy this, however, by mixing in some potting soil or organic matter (like compost or peat) into the current soil in your pot. With adequate care, the plants should do fine. There are many ornamental grasses suited to your region. Here are a few suggestions: Maiden grass: https://www.gardeningknowhow.com/ornamental/foliage/maiden-grass/growing-maiden-grass.htm, Japanese forest grass: https://www.gardeningknowhow.com/ornamental/foliage/japanese-forest-grass/golden-japanese-forest-grass.htm, Northern sea oats: https://www.gardeningknowhow.com/ornamental/foliage/northern-sea-oats/northern-sea-oats-grass.htm, Fountain grass (look for one particularly hardy to your area). In Zones 5-9, fountain grass is considered perennial. However, they seem to respond better in the warmest areas of Zone 5. They can survive in the ground and come back as long as it does not get colder than 20 F. Here is more information: https://www.gardeningknowhow.com/ornamental/foliage/fountain-grass/tips-for-care-of-fountain-grass.htm