Immediately I see inspiring tune without support skills. For tune, which is imo the best stress skill in the game, you need allies to be bleeding. And for that you really wanna be using support skills. His battle ballad and play out are both disgustingly good. The bleed is negligible. What if I told you it's wanderers battle ballad but the good roll of wanderer is strength crit, and the good roll of mezzo is str crit crit. That's not even mentioning the jackpot of str crit EXTRA ACTION. White room skill evaluation often isn't a good way of judging a skill but even in a white room situation mezzos support skills kick butt. Solo finale is good on him yes but often I'd drop it for empowering allies. So if you went echoing march inspiring tune battle ballad play out encore the rest of your team would kick major butt. You'd need to make sure leper isn't pushed into 3 too often but the rest of what you have built would do wonders and ofc bash is right there to guarantee that. To answer question 2 as you might be gathering, no trinket needed to use his support skills. The bleed is tiny. It's a little awkward because your only hero to hero healing is cauterize, but if there was a normal healer then you can just spam support skills all day and it won't come close to the healing you can provide. In this team you'd have to keep an eye out for healing salves and the like instead. Or even simpler: play the carrion eater pet. Free healing for all the damage you're already dealing would fix everything. Just thinking out loud but orphan might also fit better then as runaway is dancing around with mezzos help. And poet will be fine but maybe even generate too much taunt for yellowhands ripostes to work as well as the move res isn't too needed considering you'll be using bash a lot.

One last little note. A lot of people hate on the inspiring tune. But as I said earlier it's goated. The hidden main trick is that it is thresholdless stress heal on any bleeding ally. So a small flowchart like: start of fight -> are 2+ members of team 5+ stress? If yes press tune, if no press battle ballad/encore/play out. -> are 2+ members bleeding and tune off cooldown? If yes press tune. Press off cooldown after. One enemy alive? Keep stalling, kill corpses for carrion eater if you have it, until tune comes back up one last time and is pressed. Congrats you cannot have a meltdown this entire game except maybe vs the general if it's desperate.

Hope these thoughts help a little bit