I'm no dixieland expert but I generally agree with everything you say. If a midi tuba can't meet iRealPro's high standards, better to avoid it. And I definitely wouldn't want a horn section with collective improvisation! For my own purposes, just a more simplified bass/drums style would work well for both dixieland AND Western swing material. Actually, the current Bluegrass setting (that two-beat bass does somewhat mimic a tuba) would suffice, if there were drums (similar to Gypsy Jazz) on the Drum track instead of mandolin. As for the third instrument, being a pianist, I mute that track, but maybe "stride" piano for others.

Re Count-In clicks, I think gentle drumsticks would greatly benefit all of us who use iRealPro on live gigs. And perhaps in the future a way to adjust the volume.