The two problems are sensitivity and pain upon bite-release. The first can be caused by several situations including 1) over brushing, 2) too little brushing/care of teeth/oral Hygiene 3) morphology of the jaw-bone-teeth among others. Daily Prevident after brushing will help, but it may take 6 weeks or more to notice the reduction in sensitivity. There are topical Desensitizers your Dentist may chose to apply in certain situations. Periodically some dentin/root coverage with a GIC (filling material made for those root surface problem areas) may be an option in select severe cases. To answer your Question about the in office FL- treatment, it is always better to get the in office FL- also..... That being said.... If you have to chose one or the other.... a daily FL- will certainly be a huge advantage especially in the event of sensitivity.
For you second problem with the bite-release and then pain, your dentist is handling it well, a crown is the treatment of choice. However, if symptoms worsen, you may have the crack extension into or too close to the nerve and you may have to have
root canal therapy at a later date.
You are wise and your choices will last a lifetime. Solving the problem is only the beginning. Think of how you arrived here and see if there is anything that you can do to avoid further Dental troubles. Sometimes chewing things like ice, popcorn, very hard or very chewy things can crack teeth. Also keep those dental visits regular, and thirdly avoid over brushing. Best of luck!