If the PM corrects an amount due in favor of the Employer, are they entitled to interest on the corrected amount (over-payment)?
Clause 51.3 deals with interest on corrected amounts. The clause is silent on which party pays therefore can be either party. In the context of the second bullet point this makes sense as adjudication / tribunal decisions could go either way.
Note that the clause is very specific as to the things that will attract interest (mistake, compensation event, adjudication / tribunal decision) therefore not all corrections of an amount due will attract interest, you need to look at why the correction was needed to determine this.