Python PowerDivergence

I am no statistician but I was asked to provide some quantification of the goodness-of-fit of a theory. This theory only uses one parameter and that parameter was fixed (not a fitting parameter), so the values are predictions.

The plot of the fitting is here:

Here are the results from PowerDivergence

And here are the residuals:

How would one quantify the quality of this prediction, given the p-value=1?

How would one compare this theory with another one that provides similar goodness of fit results but has 6 extra degrees of freedom (mine had none)?
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