I think we should further discuss two topics with the hand above. Was it really a bad idea to lead the 2nd bower on trick 2?
You know only the KH & AH are unknown trumps. Thus, your partner could have the AH as well as the dealer. It's next and S3 should have another heart and maybe even two. A lot also depends on who that Dealer is? You know S2 has no more Diamonds so I see no issue with S1 leading a spade or club on 2nd trick to hit your partner's void so he can trump then lead a diamond for S1 to over trump the dealer's trick. Just a chance you have to take. You can't get euchred as S2 has shown that he has no trump. I say the probability of S4 having the AH is no worse than equal that S3 has AH, and depends as much on the tendencies of S3 & S4. 1endancies? If S4 is bagging he could have AH/KH and leading the 2nd bower on trick 2, you could get euchred.
Now to the Pop Quiz? After the 2nd lead of the the Jack, all trumps are known, and yes S2 played a low club. Obvious that S2 has no more diamonds and might as well have a doubleton club and a doubleton Spade. You don't know what your partner has and that is what is critical. So for me it is such that S1 should actually lead his other trump card. What does this do?
It allows forces opponents to slough their weakest suit and allows S3 to sort his hand and gives S1 a better idea of which suit to lead on trick 4 - spades or clubs.
IRISHWOLF