How can it be?
 

How can an almost direct quote(i should have said "plate" instead of "game" umpire) from the rule book be almost true? :) I agree that the plate umpire has to accept the reporting. I assumed, as apparently the rule book does as well, that the umpire has accepted the substitute.

Suppose the coach wants to put in his flex to bat for someone other than the DP? Suppose he still insists on making the sub after you informed him that is illegal? Do you allow the sub and then wait for an appeal by the defense?

Absolutely. What choice have you? For one of the coaches, it will be a DMC. It might be interesting to see for which coach. ;)

True for ASA. In FED the umpire can discover the illegal sub.

So tell the coach "no;" when he insists, write down the change, point to pitcher and say, "play," call time, call the illegal sub out and send her to the bench.

WMB

Of course you are not going to do that; you are going to talk the coach out of the change. But its fun to hypothesize. :p

You tell the coach "no". And it really doesn't make any difference, what the coach insists upon, it is up to you to accept the change. Just pull a Nancy Reagaon and "just say no". Eventually, you will toss the coach, explain to the assistant why s/he cannot do that and all will be right in the world :D