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Thanks for the answer, firstly I'd like to stress that in my re-reading the question on Jech I found several mistakes in my question here and corrected them, one of those was weakening the equality to a weak inequality. Secondly, my biggest problem with this question is that the intended result is not to have some transitive model in which $2^\kappa = \aleph_{\kappa + \beta}$ but rather the original model.
I think that I didn't fully understand the idea of some function $f$ such that $[f] = \alpha$ for some ordinal $\alpha$. The so-called confusing points which you mention are actually fairly clear to me.
When you say complete, do you mean metrically? Because that'd be obvious since we define metrics using the reals. Or do you mean in the sense that it is a complete order (i.e. all the Dedekind cuts are realized)?