Spoilers are weird. Thank you, anyway,

She's essentially walking deus ex machina in certain situations

So is Yugi Moto. He has only ever lost when he decided to(as far as i know, i stopped watching the later seasons).

Her ability would know the outcome ahead of a hypothetical flip ahead of time.

Yugi could, and would, manipulate the flip. His powers would allow that, and would allow him to cause a truly random event.

If there's any course of action that leads to victory, any at all, she'll take it.

Unless she has some feats predicting the positions of quantum particles, i don't think you can claim that probability does not exist for her.

If she has feats predicting truly random events(not a coin flip or a dice roll, more like being able to predict the decay of an atom or the spin and position of a sub atomic particle) then you could make that claim. As i understand it, she has only predicted natural phenomena on a Newtonian scale, which is essentially deterministic, which it why her power works in such an adamant way.

I have not actually read Worm, but if you can think of a time where her abilities had to factor in a quantum event that was truly random, then i concede. Otherwise, to assume probability does not exist for her simply because her ability has worked so far in a newtonian way would be a no limits fallacy, i believe.

If she can beat the Hiesenburg uncertainly principle, she can beat Yugi.

Let me ask another question, if contessa had to look at a schrodinger's cat experiment, what would she predict? if a particle was in superposition, where would she say it is? Assumeing her goal is to be correct.