Inverted minors are a homemade system? Ouch
Ok presuming your style is to respond 1
♦ on north's hand, which I think is inferior but fine, the first mistake was north's 3
♣ bid. He has an excellent hand with excellent support, and there is no reason in the world not to jump to 4
♣. Then south, knowing north wouldnt bid this way with bad clubs or bad diamonds (no need to 'assume' the honors, as the bidding shows them), would be able to keycard and place the hand in slam confidently.
As the auction went, south's 4NT bid was a huge mistake, because (as occured) he had no idea what to do over partner's reply. Look at poor unlimited north, about to force the hand to slam but then dropped in his own keycard reply! Over 3
♠ south could bid 4
♣, 4
♦, or 4
♥, whichever suits your style. Another way to think about it is after his first 3 bids, south has shown very accurately his strength and shape, so north knows a lot more about south's hand than south knows about north's. Therefore it should be up to north to take control and place the contract (the way the auction had gone to that point.)
So 3
♣ was the first big mistake, but 4NT was even worse.
And that makes my recommended auction 1
♣ 1
♦ 2
♥ 4
♣ 4NT 5
♣ 6
♣. Then at mps I would convert to 6NT as north, taking the chance that partner has a singleton diamond but improving my score most of the time on a hand where I expect the field to be in slam.
Please let me know about any questions or interest or bug reports about GIB.
East deals and passes. Opps silent