Interesting idea. Do you have a source to back that up, or is it conjecture?
That does seem to be the only logical way to have 100% accurate predictions 100% of the time, but it does fall prey to the problem of conflicting or interdependent prophecies that was discussed earlier in this thread.
The existence fo time is known in the same way we know anything else (other than our own existence, which is all we can truly ever know), observation and deduction. In a more scientific vein, there are a great many things we observe happening that are impossible to explain without a time dimension (note that 'dimension' is being used here in a mathematical sense).
Of course, we already know that time functions as a result of the actions of it's occupants, rather than the other way around (in our world, time flows as it will and we respond to the changes that occur because of the flow of time). Still, time does exist in Erfworld, regardless of it's variable rate of flow.
A simpler explanation is that predictamancy is closely related to mathamancy, and merely 'calculates' the actions of the various units. After all, predictamancy is right next to mathamancy as part of Hocus Pocus (fate axis vs. numbers axis). If erf were a computer game I would say that predictamancy runs through the AI routines to determine what each side would do when.
I would say no, or at least not if erf units relate to their world the way we relate to ours. That is, a predictamancer would interpret their prediction through the lens of their own experiences and biases, subconsciously shaping it to fit their worldview.
I don't think that her supposed predictamancy has failed.
One weakness of Wanda's is 'Capacity for Hope'. Who can have less hope than someone who knows the future with 100% accuracy? Even if it's only parts of the future...