It's been answered, but for emphasis, Esuna will not cure Charm, nor its countdown debuff. You need to heal the target to full BEFORE the charm takes effect and that'll remove the debuff. That said if one DPS gets charmed, it won't be for too long, but it's not something you want. I do not know about Silence, but I will try it tomorrow if I can. My bard is only iLv64 though so I'm not too sure about taking that in there yet...
Siren's charge is ridiculously wide. Much much much wider than Ifrit's charge. You almost have to be on the extreme edge of the arena to dodge it. It's better to assume she's gonna charge when you see her disappear, so just hang out in the middle between the center and the edge, but then haul ass back into the center if you see her. Her special move has a surprisingly long charge time, so you should be okay.
This fight is very easy as a healer if you keep all party members at full health at all times. Screw worrying about overhealing, just do it. You won't be wasting that much MP anyway because the adds don't hit hard, and a divine seal Cure II/3 or a Physick + Embrace will heal up a Siren charge quickly. Most Scholars also forget about Rouse. Don't. She will usually put the charm debuff up after a charge, so get that healed up before she can.
Make sure that Bleeding song is removed the *instant* she applies it to the tank. The *instant.* If the healing debuff gets off, you and the tank are priority. Esuna/Leech them first. Note that this is much easier as Scholar because of Lustrate, but WHMs are fine. Swiftcast Raise if necessary, but Divine Seal Swiftcast Medica II's Regen will actually help more than you'd think even with that debuff. Thing is you're likely gonna piss off the adds at that point, but that's what the DPS are for.