Why do we expect Type Ia Supernovae to be brighter in a decelerating Universe than what has been observed?
Observed brightness (or magnitude) depends on the distance between the source and the observer. The further the distance, the fainter we will observe given the same source.

If the distance r1 is measured between the source and the observer given the accelerating universe, then the distance r2 measured given the decelerating universe would be r2 &lt; r1. Therefore, this answers your second question. For the first question, the answer is directly followed the current model that predicts less matter density given faster expansion of The Universe.