I do kind of agree with you here. I don't think she'll betray Mathilde as Heidi understands it, outside absolutely extraordinary circumstances (like a case of Ranald vs Mathilde).Am I the only person who feels Heidi can be trusted?
Like, yeah, she's a professional conwoman, but I feel that she operates schemes on a long term scale with enlightened self-interest as a goal, and that kind of makes her... hmm, not predictable, as much as we know what levers move her. She's not going to damage the Empire, or hurt people, or betray Mathilde, because there's no benefit for her to that, and I trust her based on her knowing that, if that makes sense? I trust her to always act to her advantage, and I don't see how that can turn her against us or what we stand for at this moment.
However, I do believe that what Heidi considers acceptable and what Mathilde considers acceptable are two different things. I'm certain that Heidi would screw the empire for the sake of Ranald if the pay-off would be large enough, no regrets. Mathilde? Only if she could twist her mind to justify it falling in line with her oaths (Example: Sacrifcing the Empire to really hurt Chaos fits the imperial ideal of Fuck Chaos, and therefore is ok).
Hell, I can fully imagine her doing something she thinks would please Mathilde that really wouldn't. Disgrace a business rival through dirty means as a birthday present, for example.
I do agree that there's a good chance she portrays herself as more ruthless to impress the scary grey magister with the huge sword. That she got Shallya's blessing is a good indicator for that. But until we know more about her, I'm not sure about that. Plus, the above point still stands.To be honest, Heidi always seemed like that jerk with a heart of gold who constantly does good things but then denies having a selfless motive for doing them.