"We have taken risks before, and therefore should take every risk available to us" is not an argument that's ever going to convince me of anything.
It's all a measure of risk vs reward. Some risks are worth it, some are not. Non-mist battlemagic is firmly in the 'not' category for me, as clearly Mathilde can consistently function extremely well without it.
If we decide we do need more battlemagic, just inventing our own that we can cast as FC instead will always be worth it.
I feel like you are misunderstanding the actual argument several of us are making for battlemagic.
The argument for learning battle magic is thus.
There is a situation happening on the battlefield. For the sake of argument, we'll say that a bunch of units are charging at our exposed flank. Without Battlemagic, your chance of losing the battle and/or dying to the charge is say 80%. With it (again for the sake of argument), you can solve the situation and bring down your chance of losing and dying to say 5-20% range for rolling a miscast, followed up by rolling very badly on what type of miscast.
Learning battlemagic let's you make that change of odds from 80% chance of death to 80% chance of life. The cost is taking a risk of consequences upfront (which can be extremely heavily mitigated via the gambler). It is not "we should take every risk available to us". It is, "we trade a small risk upfront for the ability to take actions on the battlefield that have much lower overall risk of death".
Now to quickly speed through the common counter arguments:
"We can train swords instead." - Becoming a sword grandmaster doesn't let you scythe down large numbers before their charge reaches you. Most battlemagic is intended to counter different things than you'd use say a Knight Grandmaster to counter.
Mathilde can always escape with Smoke and mirrors. - There are battles where Mathilde will not want to escape and leave her allies behind.
We can invent mist based battle magic instead. - Possibly, though even with the staff of Mistery I'm skeptical that 'invent a new and powerful battle magic over several actions' is actually overall safer than 'learn this one codified and well understood existing spell' even if any individual miscast might be less dangerous. Also AP hell. Just AP hell.