[Callisto]

It looks like you're going all out for Charisma & Haste, which isn't bad at all. CHR affects your enfeebling songs proc rate. Something to keep in mind though is that total CHR more than say.. 110-120 is about max needed. At that point you would want to include a combination of singing and instrument skill in order to floor your resist rates. And for casting Elegy, an Earth or Terra's Staff is a must.

Yes and no there...debuff song potency and duration are set, it's really just a matter of landing it(like with Ninjutsu, you can't increase potency but you're guaranteed it'll always be a certain level). A CHR/Skill/MAcc build is good, basically a good rule of thumb is take whatever the largest boost you can get, and IMO MAcc < CHR or Skill in equal amounts in regards to debuffing.

While we're on singing & instrument skill, you should have a gearset for that as well. Party buff song effects benefit greatly from having these skills at their maximum (right now the cap for combined Bard skills is believed to be about 490 before before merits and instrument bonuses). Some good pieces to use for this are Relic Head, AF Hands (NQ or +1), Minstrel's Coat, and Wind/String Torque. Chanter's Staff and the skill earrings/rings tend to be luxury, so I'd hold off on those unless you wanted to experiment with the skill caps.

Almost every song will hit their potency hard cap with just capped skill + a few decent chunk pieces, Torque + Choral Cuffs +1 would do the job on most. The only song shown to have higher potency tiers at higher skills(500+) is March, so how in-depth you go with a Skill+ set really depends on how importan the extra 1-2% of Haste are.

The song cast time reduction (aka Allegro)

Isn't that one of those pills that makes yer peener all happy? >.>