5C was the slam killer.
[ I still don't understand it ].
2D! established the GF, so 4C ( not 5C ) would set the tone.
Heck, Opener, now knowing of a DBL-fit could just bid 6C.
Or conservatively, go 4NT over 4C...
a) pass 5C , if no Ace reply ( Sp obviously )
b) bid 6C, if 5D replied.
However, even if no Ace, if North had the remaining K ( Diam ), it could provide a discard for the losing Ht ( if no hQ ) ;
Or, if 2Q's ( no A or K ), the sQ and cQ would be nice and slam still has chances.