However, during challenging times, you must look into your target market (tenants) and predict their behavior or feasibility that they will pay you.

The rent striking behavior you're describing is not going to fly in many markets.

Any suggestions on if I can fight an excessive fine and what else I can do to get her to change behavior?

Be factual and concise and don't name call or disparage the tenant,  just describe their actual behavior.+1 on groups like WA LL association and RHA  (rental housing association of WA)

I'll have to keep an eye out for patterns of this kind of behavior. 

I am not advocating for or against this kind of behavior, merely pointing it out.Texas is a little more conservative with property rights of the owner.

I feel your pain, by early March I had a tenant that was becoming an issue due to chronic late payments and other behavior that was concerning.

You are basically rewarding bad behavior and in this case he gets greedy and asks for more.

None of those behaviors fit my definition of sweet or cooperative.Here's what I would do if I were in your shoes: if you haven't already served written notice, it's too late for January and you'll have to be satisfied with terminating their lease at the end of February.  

I do not intend to drag you through the dirt but inappropriate behavior in a learning environment needs to be called out.