As a licensed attorney and a MSN, I work as a Legal Nurse Consultant for a major insurance carrier. I can imagine the salary offered by the law firm seemed very low for you. However, without a legal and/or claims background I do not think that with just a "course" or certification course that you will find yourself marketable at $150 an hour.
Could you work at the law firm part time on the days you do not work 12 hours? The experience you will get there and they will pay YOU $15 to teach you. Better than paying any course in my recommendation.
Law firms either pay consultants who have extensive experience in a particular clinical area that relates to that case which can make it a lonely field if your only experience is in say one area. An online course will not teach you the legal and claims arena. It takes exposure and experience mixed with your medical background to usually be a successful LNC.
Good luck. I say find a part time gig at a law firm or ask to do some record interpretation for them, for peanuts if need be. Without the experience, certification won't help......they will not even begin to pay you what you making now.
I love the work. But rather be a nurse practitioner. Nursing is more fun than lawyers.