while i dont entirely trust her, i will say that "Ambition" is an extremely vague term in this context. Ars assumes that ambition is strictly political ambition, ie they want a lot, ,while it could be that they are willing to do a lot to get what they want. He assumes she wants a lot politically, mainly because her political stat is already so high 77/100. But we can compare that to Ritsu who is at 90/100 with 21 ambition or Rosel with 50/95 politics also with 21 ambition.
If her true desire is to marry into a family and where she will be loved by her spouse, which could very well be true because lets face it, shes a young girl born into a noble family, her motivation makes sense. The schemer mentality only makes sense if she is also a reincarnator, or if she is that good at playing people like a fiddle like Ars is afraid of.
I think its more interesting to see Ars flounder and struggle because he interpreted his own skill incorrectly, than to just assume his fiance is out to get him, but if im proven wrong down the line and she betrays him, that will be an interesting story for our MC, especially when everything has gone perfectly for him so far.