I'm not claiming to be right or wrong, as with many sequences there is room for disagreements, but this seemed to me a preventable disaster more so with the 6/5 hand than with the opening hand.
My views are these: 2N expressed the extra values. Partner's 3D is a delicate probe of some sort, most likely in this auction to be a weakish 6/4, 6/5 or 5/5 with some doubt. 3S showed an interest in a suit contract without ruling out 3N if partner held some hand like: AQxxxx, x, Q10xx, xx but allowing a pass if he held something like: AJ10xx, x, Q10xxx, xx. IMO, with no doubt about the spade quality and length, partner missed 2 chances to bid game: directly over 2N (we go out of our way not to bid NT with singletons) or at the very least over 3S.
Partner's views: 2N may only be looking to improve the contract. (later rethought and altered to extras but not necessarily great extras). 3D pinpointed the problem he thought he had with his hand, the losing diamond cards, and therefore the correct bid over 3D was 4S.
It seems that partner's approach places a higher priority on stopping short of game on those few times it is wrong than on bidding games when there appears to be a reasonable play, whereas my approach is to bid the best-seeming game contract and only stop when the cards tell us we have to do so.
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S W N E
1♥ P 1♠ P
2♣ X 2♠ p
2N P 3♦ P
3♠ P P P