I very nearly forgot to make Sarah blush in this comic. That would have been a tragedy.
The changing things affecting accuracy beyond what's directly being changed makes sense. Or, at least, it does from my perspective, and I'm writing this nonsense, so therefore it DOES make sense within the logic of the EGS universe, so there.
Sarah's spell creates a simulation so realistic that it can be indistinguishable from reality. It is, however, still in some way her imagination. It's magically augmented to maintain a certain state and for things to be moved around while maintaining accuracy, and one body at a time can be controlled (hence the body swapping) without affecting accuracy.
Once her imagination is allowed to makes changes, however, that accuracy starts to give. It's difficult to imagine with laser point precision. One thought leads to another, and bam. The magic keeps everything feeling real, but suddenly she's changed more than she meant to. If she makes changes to check for a mark she thinks is going to be there, odds are it's going to be there even if it isn't really there on the actual person.
So hey, that's one limitation in terms of information gathering. It's not much of a limitation if she's clever about it, but it's there.