1. Keep a notebook. Or a spreadsheet. It's not hard, it takes a few minutes to enter data, and maybe a few to look it up. If you have multiple directors, make sure they all have access to it.
2. Don't make a ruling harder than it has to be.
3. Know what the rules are in your club.
4. If the rules are unwritten, as they are in the clubs around here, lobby to get them in writing. Even if it's a simple one-liner like "ACBL Alert regulations apply except that a 1NT opening with the range 15-17 HCP need not be announced." Make sure the players have access.
5. Make sure you have a law book with you when you're called to the table. Read your ruling from the book. Even if the players complain that "we don't have time for that". Make the time. If you need to delay the movement, delay it.
Are ACBL convention charts (which one) and alert procedures in effect in this club? If so, then there were apparently three infractions in the auction:
1. East failed to announce the range of West's 1NT opening. Probably because he heard a (false) rumor that the announcement is no longer required.
2. South failed to announce that North's 2
♥ bid showed spades. Probably because he forgot that's what it showed.
3. West failed to alert East's artificial 2NT response. Probably because he forgot they were playing lebensohl.
The first thing the director should do when called after the play (as you were, right?) is to read and explain Laws 20F4 and 20F5 to the table. Make sure they understand that before correcting MI they
must call the director to the table. In this case that means the declaring side should have called the director
before the opening lead was chosen. The defending side has to wait until the play is over, but still must call the director before correcting the MI.
Now we get to Law 20F6: "If the Director judges that a player has based an action on misinformation given to him by an opponent see, as appropriate, Law 21 or Law 47E."
Now the director should investigate why North bid 2
♥ and whether not getting the NT range made any difference to his choice. Assuming it didn't, we continue. Why did East bid 2NT? Looks like his plan was to bid 3NT later to show a game raise with a heart stopper. So the question is, would he have bid 2NT if he'd known North had spades? Or, does he consider the stiff king to be a stopper? I would guess not, so he might have bid 3NT directly. Would West have passed this? Sure, because he does have a spade stopper. So they'd have ended in the same contract.
Now we get to the hard part. What was West's line of play? Apparently nobody at the table can remember how the play went, except that North led a spade, presumably the Queen. Still, assuming the players haven't mixed their cards, you can reconstruct this. Have them put their hands back in the board, carefully preserving the order of the cards. Then tell them to get on with their game; you'll give the final ruling later.
If it looks like West took a line of play that assumed North had hearts and not spades, then the MI affected the play. Law 47E says the director "may adjust the score". This means you look at law 12, and ask "was there damage?" if you judge that West could have made 3NT with correct information, you have to ask how many overtricks he might have made. In this case you should probably end up with a weighted ruling. Let's say it looks like there's about a 40% chance that West would still have gone down 1 with correct information, a 50% chance he'd have made 3NT, and a 10% chance he'd make an overtrick. There's your weighting. You should look at the matchpoints for each of those scores to be sure. It may be that all three results would give the same matchpoint score, though that's unlikely. If it so happens, though, then the final ruling is "violation of the Alert regulations not resulting in consequent damage to the declaring side per Law 12C1; result stands". Or "violation of the Alert regulations resulting in consequent damage to the declaring side per Law 12C1; score adjusted to M matchpoints for EW, N matchpoints for NS" (where M+N=the total possible matchpoints on the board). Or you can tell them how you weighted the raw scores to come up with the adjustment.
Is all this time consuming? Yes. Does that matter? It shouldn't. The director should, of course, try to minimize the time impact on the players, e.g. by instructing them to get on with the next board while you do the time-consuming part of the ruling.
NB: you may need to consult with West (and possibly North and South) as to how the play might have gone with the correct info. If time is tight (in club games here, time is almost always tight) you'll have to delay that until after the session.
This is a judgement ruling, so you need to keep Law 83 in mind: "If the Director believes that a review of his decision on a point of fact or exercise of his discretionary power could well be in order, he shall advise a contestant of his right to appeal or may refer the matter to an appropriate committee." There are almost certainly directors who will not believe that this law could
ever apply to one of their rulings. They're wrong.
If the players
have mixed their cards you're not going to be able to reconstruct much. In this case I'd remind the players of Law 9B2: "No player shall take any action until the Director has explained all matters in regard to rectification." Assuming the director call came immediately at the end of play, that includes mixing their cards. And then do the best you can without that information.
I don't think I'd issue any matchpoint procedural penalties, but I might warn them that particularly violation of a "must" law is a serious matter, and make a note to give them a PP if they repeat that particular offense.