Karak Eight Peaks may be the single largest, most impactful positive change worldwide if it works out. I would see it in the same ballpark as Magnus founding the Colleges of Magic. If it heralds a revival of the dwarven race, (not likely, but plausible) then it may be even more significant. I don't want to take away resources from that. In fact, if Mathilde happened to receive a similar boon from another group, I'm fairly certain that many would vote to spend it to help the dwarves. Some posters have already been arguing to spend speculative elven favour to mend their relationship.
I think it would be entirely in character for Mathilde to tell Belegar that the biggest favour she could receive would be for him to remain a force of careful, deliberate, and most importantly positive change. The most important way he can do that is by ensuring the success of K8P, which will have so many consequences it seems ridiculous.
Yes, but this is the part of the quest where people act incredibly arrogant. Really, I cringe at some of the implied superiority in the discussion about the poor dwarves and their obviously defective ways and how couldn't we use our boon to try and force them to be Better (tm) in a way defined by the superior wisdom and morality of us quest participants.
Just ask for something useful to Mathilde. Seriously.