Certified GKH Gardening Expert
For the fern's optimal health, it is probably best to not trim the fern except that you can remove any fronds that look dead. It will recover nutrients from the damaged leaves, shed any leaves that it doesn't need anymore, and then regrow when it is ready. However, if you want to trim it back to improve its appearance, you can remove the damaged fronds as long as it still has some left.
Yes, you can fertilize around the outside of the plant.