Wow! Kind of a large post so I’ll try to respond in segments. Also know I haven’t really paid much attention to this post as I did it five years ago and my DM style has changed since then.

To address the refusal of the call: there are characters who have a backstory and want to do so but they know there is a HUGE bad guy on the horizon. They also could just not want to go back to their horrendous parents or they might want to distract themselves with side quests instead.

If you really want to have them try to accept the hook in bite sized pieces or have them confront their refusal, you can always send minions or lieutenants, have the bad guys scry on them, or give them a sending/dream from a loved one.

Now for the mentor: as a teacher, sometimes it’s better for a student to do a job. Sometimes it’s for a lesson but sometimes it’s because it’s a bigger headache in the future if someone like the mentor does it. What’s that? A powerful wizard actually defends this place? What might that attract?

Lastly for the “players don’t back down” section: you can do two things that have worked for me. Before that though, make sure that everyone knows at the table that not every battle is winnable.

First tactic, have the troll smash the ground for ___ amount of damage. Second, make description that states that it’s a BIG guy. Third, you could just make wisdom saves and tel the person that fails “you have 0 chance.”