You're not wrong, but that wasn't exactly what I was getting at with saying that. So I'll explain. It's safe to assume that an agent isn't going to directly intentionally act against the wishes of his client. If the list is there, it's there, if it's not, IT'S NOT and it'll eventually come out in the wash if they fail to provide. Them failing to provide the proof is eventually going to come back to them, as this Dadonov mess proves unquestionably. Does the agent failing to provide it hold up in court ALONE, no it doesn't, but it's normal to assume the agent provides either the proper proof or acknowledges that they do not have it, the logical exception being if the agent messed up and didn't file it properly, at which point feigning ignorance is covering up a failure to act according to the contract with the client... again it'll eventually be exposed if the player gets traded to a team on their no-trade list.
So, (I'm not saying this is what happened, but providing a hypothetical) if Vegas asked Ottawa to show proof that the list is invalid or that the list exists, assuming you can't talk to a player who isn't under contract to you, Ottawa would then have to check their records, if they had no records, they can approach the agent to show them the acknowledgement, if at that point the agent fails to do so, Ottawa should go back to Vegas and say "we can't provide this information at this time" (A great reason not to try tricky stuff so close to the deadline). If that doesn't happen and the agent provides it, you have it in writing and have done your due diligence and can make the most informed decision. If Vegas still makes the trade here, they'd have the proof they need and the Anaheim deal never happens.
Now, if the proof never shows up and not doing their due diligence, Vegas moves forward anyway, you have a different scenario. If after the Anaheim deal the agent claims Dadonov had Anaheim on his no-trade list, it turned out that they submitted it late, or through improper channels and didn't get an acknowledgement, OR if the agent claimed ignorance, Dadonov is a Duck. Ignorance only works so long as the proof isn't needed at some point down the line and I'd bet no agent thinks that's a good idea in the case of no-trade lists. Since the list WAS there and submitted properly, Dadonov is a Knight.