Answer: Trimming won't hurt it. The deep mulch will help protect the base of the plant so that even if the top freezes there is a chance the roots and lower trunk will survive to resprout.
A better insurance policy is to cover the plant and provide a source of heat on a cold winter night. A string of outdoor Christmas lights or a couple of indoor light bulbs is often enough heat to make the difference underneath the cover. Just take the necessary precautions to prevent a fire hazard or a short if the weather should turn wet.
Good luck with your hibiscus!
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