Guillaume is invisible, right? He follows Estaban and Lilly, and witnesses Old Man Martain send Lilly for water and wisper to Estaban. Sponting a spell will be a challenge. Being invisible incurrs a penalty for no gestures, and you need to use no voice if you want to remain undetected.
Bienvenido smiles, and waves his arms, a chicken wind in one hand, a mug in the other, and the mouth still half-full of food
Aye, ma'ied life did 'onders t'me. Ande childen! A man don' know lif 'til he hachen't had childsh.
He blabbers a little about his wife and children, and then comes back to the buffet.
This isn't necessarily a problem. If ogres are unlikely to try climbing over an abatis, they're even less likely to climb over a burning abatis. Vocis adds with a smile, Just try not to reduce them to ash too quickly.
Bringing the two hordes into close proximity also may set them against one another, which makes our job that much easier.
Vocis just nods.
I can also dissuade some of them from approaching the gap in a rather chaotic fashion. What passes for 'ranks' in this force should be broken and confused. Vocis turns to Vibria. Do you have the means to cause one or more targets to burst into flame? Like this... and he casts Painted with Fire on some nearby rocks. This is just an illusion and a temporary distraction, but if a few targets are actually burned alive by the flames, the effect on the horde should be improved.
Do you want me to retrieve Guillaume and Esteban? If not, I think we should at least let them know what's happening. The plan we're discussing won't be quiet.
Inform them of what is going on, but leave the decision up to them. From the bottleneck, the town won't hear much. They will see plenty though, plumes of smoke will be seen for miles around.
Yes, he's invisible. No, he doesn't care about gestures and voice. He just doesn't want to have to shove his way past anyone.
You are good to go. Try a spell 
Guillaume casts a MuVi spell to alter Distant Shimmer of Obvious Mysteries [CrVi10] cast upon the man, both silent and still. He'll be changing both Technique and Form to InCo to discern what is physically wrong with the man. (I'm nowhere near my books, but a R: Sight version of that low-level physician's eye spell should do.)
The MuVi spell is (47 +/- aura + roll)/2 with 1 Fatigue level. Can't get into Invisible Castle right now, so I'm going to assume level 25 is a good estimate for now. This is to alter a level-10 spell at R: Touch, with an extra +1 for the second Art being changed, so I think he's good if he doesn't botch (botch dice: base + 2 - 2 - 3 = base - 3). He really doesn't need to worry about Distant Shimmer of Obvious Mysteries because his casting total is so high above it and he has it mastered so he's got 6 botch-dice reduction to a possible 0. Mark, do you want to handle that MuVi casting roll and potential botch?
Wait, what the heck are you doing? Casting this spell to cast that spell to grind the coffee and cast that at...
Just to save us some time. There is nothing really wrong with the guy other than old-age.
Looking at the numbers, as far as I can tell, the math all fits. But I really should make you roll dice for something that complex. But I know Invisible castle is down. There are a few other roll-sites, I am looking into them.
So at home I rolled my polyhedral d10, came up with an 8 then a 9. So it all works.
They guy is 72 years old, arthritis, lungs are weak, hair falling out, and irritable bowels.
TMI? 
Astrid moves toward the man after Guillaume gets out of the way. She glances over him and replies rather late, "It seems just to be old age and the frailties that come with it. I do not think you need fear anything."
And a concentration roll to cast two spells simultaneously.
No, that's not how it works - it's not two simultaneous spells. You cast one which lasts one round and follow it with another. You cast the MuVi one round with a Momentary duration followed by the formulaic spell the next round. I believe Mark allows fast-cast spells and normal spells in one round (which I believe the rules officially do despite the original intent otherwise), in which case they could be cast in the same round with a fast-cast MuVi.
Found it:
Then you can't do it at all. "It is not possible to use Muto Vim to affect another spell after it has already been cast" (p 159). If we accept your quoted clarification as nullifying the existing RAW, it still doesn't work-- your target spell is only duration Momentary, the spell no longer exists the round after you cast it, and thus cannot be a target for metamagic (unless cast in conjuction with a second magus).
I'm pretty certain that's how it's supposed to work, actually: MuVi first, then the spell it's supposed to modify the round after.
You can't cast the MuVi before the spell you're modifying, because the target doesn't yet exist.
Sorry, didn't think I needed his next sentence where he dealt with that:
This is part of why you typically use Day-duration Wizards' Communion and the like to apply MuVi to rituals. The MuVi affects the spellcasting started right after it's cast. If the next spell is not started immediately, the targeting fails. Meanwhile, if the MuVi doesn't last long enough, it doesn't modify the whole of the spellcasting.
Also, note that Momentary can essentially last one round. So from the time you finish the MuVi you have one round to finish casting the other spell.
((Do any of the grogs in town have any experience with magic? Between Veil of Invisibility, Seven League Stride, and Silent Shout, Vocis could technically get to the village and contact Esteban as long as he can see him, but he prefers to communicate through grogs both to avoid penetration issues and because springing it on a fellow magus is rude. 
Also, Invisible Castle's still down, so I can't roll for any of this.))
[color=red]"Not yet," Vibria says sadly. [color=red]"I know Pilum of Fire, I can breathe fire, and I should be able to cast a spontaneous spell to make someone's clothes catch fire."