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You should cut down the plumes at the end of winter, to make room for the new plumes in the spring.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

The pots alone may not have offered enough insulation for the Pampas Grass root system survival from the cold.
When plants are in pots they are more vulnerable to freeze and thaw.
I would wait a bit longer to see if you see any growth. Time will tell.

https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm

Yes, new grass blades should grow up through the brown, cut foliage from last year. This should happen soon. If not, make sure you have planted it in a location where it has what it needs:
https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm

Many prairies are burned in controlled fires because the fire can kill off unwanted weeds but also cause the seeds of certain prairie plants to germinate. It is not necessary to burn pampass grass, though. You should always be very cautious when burning anything. This article may help with your pampass grass:

https://www.gardeningknowhow.com/.../growing-pampas-grass.htm

Your Pampas grass will do better if you cut it down later in the year -- literally at the end of winter. If you cut it down now, then yes: you run the risk of cutting some new growth and it may not reach it's full height afterward. This article may help you:

https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/pruning-pampas-grass.htm

Summer Grasses are some of the last plants to start growing in the gardens.
These articles have more information.
https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm
https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/pruning-pampas-grass.htm

You can also check with your County Extension Office to see if they have dates of bloom time for you Pampas in your region.

https://www.gardeningknowhow.com/extension-search/