The key thing missing is that using 2NT as a puppet bid ("please bid 3
♣, partner, so I can show you my hand") [*] effectively doubles the number of hands you can show. And it means that 3
♦ *directly*, without going through 2NT, is Game Forcing. Which solves the problem of when you have
or you have
and only have one diamond call short of 3NT.
Quick overview of "standard" lebensohl. Direct:
- 2 bids are to play
- 3 bids are game forcing
- cue is GF Stayman without a stopper (**)
- 3NT is natural, but denies a stopper (**)
- double is penalty(**)
but after bidding 2NT first:
- pass or 3 below their suit is to play ("drop dead")
- 3 higher than their suit is invitational
- cue and 3NT are as above, but show a stopper
(**)More complicated here because 2
♦ shows 2 suits, neither of which are diamonds. There isn't really a "standard" on how to handle this here, I think GIB goes with "choose the cheapest shown suit as the 'lebensohl suit'". But in general this is how it works.
I've learned that in "robot games" whether it's robot indys or "free duplicate" or ... that a lot of players don't know GIB's system and don't read it, and that when I open 1NT and they interfere, or when they open 1NT and partner interferes, I'm headed for a good score just because people are going to misbid. You were right to pass; those that raised because they had such a good hand for diamonds - got away with it this time (if in 5, only if they played it right).
When I go to sea, don't fear for me, Fear For The Storm -- Birdie and the Swansong (tSCoSI)