Now apply the same logic to a situation where, say, Wanda is offered a way out of Gobwin Knob to join the coallition. Say she is so loyal there exists only a 0.5% chance of her defecting since it would be against her character. But still, repeat the scenario 200 times and you can expect one of those times she will make a decision wildly out of character and save her own skin.
I don't think thats going to happen. A huge part of Erfworld is showing how these "units" who abstractly are solely based on stats are infact "real people" with their own characters. Any character doing something ludicrously out of character and then having it justified by the story saying "Well so and so had a 2% chance of doing it by the rules, and it just so happened they rolled a 1." It would seem very very silly.

Still, interesting point, however I think you have to concede that Erf units have more free will than we'd like to think. Take Wanda, I believe her actions show she is very loyal to Stanley, almost certainly for a reason we do not know about yet. She has therefore DECIDED to be loyal to Stanley, her loyalty has not been simply defined by a roll of the dice.

Regarding the battle I think a very important factor will be this. Is the Dragon's fire breathing an area of effect attack? Or one that can only target a single unit? 30 dragons each with a powerful area attack will quickly fry the bats. If it isn't well its gonna be tough to take them down unless Stanley can spam Van De Graff every attack.