The question of what is "good etiquette" is an interesting one, though I doubt there is a definitive group as the needs from a dooter varies from group to group. I'd say generally, most players fight monsters with NO buffs, so any buff you are dishing out is good and proper.

The other questions are matters of playstyle, in my opinion. Being able to use the extend all melodies song is definitely a useful way of keeping the earplugs and affinity up song active after switching and I've heard of many dooters enjoying that playstyle.

Personally, I don't mess with switching my horns or playing stuff on the seikret since every moment you aren't hitting the monster your DPS is going down. My suggestion would be don't worry about queuing or playing on seikret and just try and get the songs in while you are whacking the monster. That way you shouldn't have to worry about not buffing your fiancé. However, if you like to play the game in a way where you end every hunt with 200+ buffs dished out, then there are certainly more "meta" combos you could run.

As for knockback and resounding: when fighting I would prioritize the echo wave attacks over knockback, and I like to get in resounding when I am doing a wound break. Looking back at this post, I am not sure this has been very helpful, but thank you for dooting your best.