A Tropical Hibiscus can not take temperatures below freezing. You are on the lower edge of where they will survive so winter is going to be a challenge. Although your Hibiscus are suffering some frost damage, the Frost Blanket may be what's keeping them alive. The blankets are meant to be put over the plants in the fall and removed in the spring so water needs to go through them.
Don't prune your Hibiscus in the fall. Those of us in cold country don't prune until AFTER things leaf out. There is always going to be some dieback on the more sensitive plants. If we prune in the fall or winter, the dieback is from where we pruned. If we wait, the dieback is what we would have pruned off if we had done it in the fall.