The strongest Dawi dispelling is based at the Citadel. It will cover the Caldera well but once the fighting moves inside Karagil, Kragg and Thorek can't see any shamans. That is a damn good reason for Mathilde to be among the throngs. Also, note that these assassination missions are dangerous for Mathilde, after she hits the target she usually ends up running for her life - the Bosses will be among their followers and keyed up for fighting. Having heavily armoured Dwarves at your side is a much better long term prospect.
Max has specialised in using silver arrows for combat as his contribution throughout the expedition. He should have no problem casting reliably from Gyrocopter. He has also stood dispel duty and the Gyrocopters are a really obvious target for enemy spellcasters and might not always be where the runesmiths can protect them.
Johann is artillerytastic.
I don't think the Journeymen have the range to add much to the fire support from the citadel. Meanwhile fighting alongside Dwarves in battle is a skill in its own right distinct from tunnel patrol shenanigans. It's a skill that will be important to them if they want to find work round here. They don't have the armour to be front rankers but they're taller than Dawi, their lightsabers have decent reach and they have zap spells too. Plus they give whatever unit they're with embedded counterspelling. Being part of a well organised unit is safer than what they get up to on the excursions with us.
The K8P throng is a better choice for them than the point of the spear with Azul though.
If Mathilde is inside the mountain, she can kill any shamans as they try to respond.
There's a military saying, 'you fight as you train'. The Journeymen haven't trained in fighting in a dwarven shieldwall, so they should be expected to be crap at it. If they want to join the shieldwall, they can ask for training and then they can do it. They shouldn't just try to ad lib it while not having the first clue about what to do.