1NT was alerted (forcing, no further questions). 2NT was alerted. The partner was asked "what does this bid promise". The response was "A two suited hand, in first approximation minors". There was no additional alert in the auction, no further questions were asked. When the dummy was faced, the TD was called. It was established that the 2NT bid shows a two suited hand with all suit combination that do not include ♠, i.e. ♣-♦, ♣-♥ and ♦-♥ are all shown with 2NT. The 4♦ showed the ♦-♥ holding. During the explanation, the player asked if I was familiar with the "unusual 2NT convention". Now the questions:
1. Was the explanation according to the laws? I was arguing that the agreed meaning of the 2NT is "two suited with the three combinations equally likely". The explanation given is more along the lines of "two suited, very likely ♣-♦". To me the two are not the same. Mentioning the minors has no reason whatsoever but it creates confusion, i.e. it is misleading. Even though it was never claimed it was an unusual 2NT, mentioning it added to the confusion.
2. If you assume for a second that the 2NT was an unusual 2NT (i.e. minors), how do you interpret the 4♦ bid? My argument to the TD was that the 4♦ simply showed a good (6+) diamond with a bad (5) club. The response was "this is complete nonsense". How would you interpret the 4♦ if 2NT is an unusual 2NT?
The legality of the explanation itself is an important question. The TD call took a long time and a board had to be cancelled. The TD ruled that the TD call was frivolous and awarded AVG-/AVG+ for that board, favoring the pair that bid 2NT.
As for the damage: North did consider 4♠ over the 4♦ but rejected it, not wanting to push them into 5♣ or 5♦ -- after all one can see only 6 tricks in spades or even less as a Jxxx by East is quite likely. Being aware of the potential ♥ fit that was not confirmed yet, he could have bid 4♠ (expecting to go down 1-2) to prevent the opponents to find the fit. In that case the 4♥ bid is not available, only 5♥. That is much harder than 4♥, thus, there is a slim chance that the contract is 4♠ doubled, going down 2. There is also a chance of them playing 5♦, not 5♥ (both makes).