I don't get this concern. I really don't.The core of Burning shadows isn't the big problem I have with it. My problem is that it treats "hold off the assault for one day and kill multiple shamans" as a given.
Personally, I think that the mercs will not stick around with time to properly reflect, that this plan will put too much emphasis on Mathilde doing everything, and that expecting us to get multiple hero kills like that is wishful thinking in the extreme.
The mercenaries are in the middle of the mountains. Walking away isn't something they can just trivially do. On top of this, the campaign has gone really well, they've had a massive party, and they will be seeing a massive amount of greenskins being melted right in front of them.* This tends to be the sort of thing that doesn't encourage asking for your pay and buggering off (back across the plains, with their own dangers) over a single day's delay.
If there was a week's delay then I'd agree with you. A week is a long time to think. Or if we try to stretch our objectives again after this. But a day, half of which is probably going to be spent nursing a hangover, seems very unlikley to cause the kind of rebellion you are predicting.
*Side note burning shadows does work on structures, just more slowly than on organics.