Some of what you wrote was difficult for me to discern. For example, I am confused by the part about how you discovered she had been searching for another room to move into but that no one had used her computer for this purpose… but I think I have the gist of it. Your main point is that you strongly suspect that she had an affair in response to her belief that you were cheating on her and now she refuses to discuss it with you either way.
If, in fact, she had a one-night-stand to punish you, she acted precipitously, childishly, and vindictively. I think most mature women, upon discovering photos of their boyfriend with another woman, would at least confront the boyfriend with the opportunity to explain himself. I’m assuming here, of course, that the photos were not sexually explicit because, if they were, those images would be highly provocative even if she hadn’t assumed they were current.
If she had an affair and believes that telling you the truth would end the relationship, which apparently is not something she wants, then, no, she most likely would not tell you. However, this is informed far more by human behavior in general than it is by Chinese culture specifically.
A hickey (the red mark on her neck) is hard to mistake for anything other than what it is. Whether she spent one night necking with some guy or actually slept with him is no more significant here than the fact that you no longer trust her—particularly in response to the fact that she is refusing to talk about the incident either way. You seem to be attempting to discount your understandable suspicions and feelings as some questionable byproduct of a cross-cultural breakdown even though you would be feeling the same way if the girl were of the same nationality. It’s not easy living with someone you don’t trust and can’t communicate with, even if the latter is partially accounted for by a cultural and language divide.
If your girlfriend is quick to react irrationally and vindictively, then this is something you are going to have to continuously live and deal with even if the two of you are able to get past this most recent episode. It is not possible to be happily coupled with a partner you don’t trust.
Please feel free to follow-up with additional information and questions.