Sorry, the website wouldn't let me post until you posted again.
It will mean suing them, unless the trust document itself defines another way to challenge them (which is rare). For instance, I saw one trust that outlined a procedure where a beneficiary could challenge a trustee by suing them in a lawsuit, or by going to a different trustee that was named in the trust and letting them make the decision. In essence, that one was an arbitration, even though they didn't use that name.
There's no case law that states challenging them would mean not complying with their requests, nor telling them that you "might" sue. You would actually have to file the lawsuit to challenge them.
Under your facts as you set them out you did not challenge a trustee's discretion.
Also, the term discretion means that it is a decision whereby they could do multiple different things and they chose one of them to do. That would be an exercise of their discretion.
If your question has been answered then I'd offer my best wishes to you and ask that you please not forget to leave a Positive Rating so I receive credit for my work. Of course, please feel free to ask any follow up questions in this thread. I want to be sure that all of your questions are answered.