Both ADP & ACP Tests fail. Must return excess contributions and excess aggregate contributions (this is in accordance with terms of plan document). Excess contributions have related match contributions to return/forfeit. Question: What impact does the "correction" of the related match contributions have on the ACP Test? Specifically, can you potentially reduce the amount of excess aggregate contributions to return based on the amount of the match related to excess contributions that is "corrected"? Any written guidance on this?

the rate of allocation of matching contributions is determined AFTER any corrections are made.
see 1.401(a)(4)-4(e)(3)(iii)(G)
in other words, distribute adp and acp to pass the test, then check to see if you need to forfeit additional match.

Let's assume (for simplicity's sake) that your plan doesn't accept employee after-tax contributions or doesn't match them. In that case you could run the ADP test, then forfeit any hanging match for HCEs receiving ADP test corrective refunds, then run the ACP test and multiple use restriction. I think that still complies with the point Tom Poje raises above.

More generally, there's nothing in the regulations (check your plan document too) that says in what order the ADP / ACP / hanging match / multiple use problems need to be corrected and you can work on them all simultaneously. When you're done making corrective refunds, just make sure there's no hanging match.

So in a nutshell, just because there were match $ attributable to the excess contributions doesn't necessarily mean that you have a discriminatory match? The rate of the match would be based on the employee deferrals after excess contributions are refunded-right?

cautiously I say yes.- if I understand you.
your statement sounds true if the ACP test was failed and match was returned as well. In other words, though I 'had' match attributable to returned deferrals, this match might have already been refunded because of a failed ACP test. so now I no longer have this match attributable to returned deferrals.
and then you get to your second point, you check the rate of match after excess contributions are refunded.