A key thing to consider here with each of the options is how many forces can Mathilde free up by accelerating progress at each site.
Her presence or absence will not change the fact that the Underways will need guarding. They'll always need guarding, and the need for guarding will grow over time as the skaven and goblins have chance to mobilise. Right now is the time when they will almost least ever need it, as the neighbours won't be ready to march out to attack at a moment's notice. It's better to accelerate reinforcement from the rest of the expedition than for Mathilde to attempt to do it herself.
Kragg the Grimm will be busy with the contents of the vaults after opening them, so won't be available to reinforce elsewhere immediately afterwards.
Of the prevailing options, only the forces assaulting the Chiselwards can be redeployed to reinforce other locations. They're the one place where Mathilde's presence can make success compound on success.
That's why she should go there. She's only needed in the other two locations if there is a crisis, but she's much more likely to be able to accelerate clearing the Chiselwards, even if they're completely empty, unlike the other two. That frees up the Rangers and soldiers to go and do other useful things.
Also, if I were a sneaky rat(goblin) bastard and wanted to get into the sealed areas and I knew dawi were coming, I'd set up in a hidden position, let them do the unsealing, then rush them in force. Mathilde's eye for an ambush could well be needed.
How would you set up a hidden position that the dwarves with their affinity for stone wouldn't find, given they're sweeping the place for enemies?
