For Meripo your best hat is Walahra Turban. You need to pull very fast. Haste speeds up songs therefore the Wally wins.
As for Opo-opo crown, it does have a wonderful +CHR bonus, but that's really not a bonus for meripo since you're fighting really weak mobs. I stick with the Walahra Turban.
As for problems sticking Lullaby, that's why you have two. If you're going up against Colibri, I've always found going /NIN and blink tanking a little more palatable than tossing Horde. If Lullaby fails, I shadow tank the new one until my next Lullaby comes up or one of the vokers graciously covers it for me.
As for the difference between +CHR and +Skill:
Magic Accuracy - FFXIclopedia - a Wikia Gaming wiki
There is probably a dCHR inflection point above which the effectiveness of +CHR is
halved, and there is probably an inflection point (assumed 200 by the referenced document) at which skill is only worth 90%. Therefore, the idea is to hit this cap with
both CHR and Skill and thereafter focus on Skill > CHR.
100 of a given attribute is generally assumed to be the "cap" everyone seems to shoot for on everything else. I probably wouldn't try pushing it much past 110 even if I was being liberal.
Assuming inflection of skill, the Demon Helm is worth 5 * 0.9 = 4.5 Magic Accuracy.
Uninflected, the Opo-opo crown is worth 7 * 0.5 = 3.5 Magic Accuracy or 7 * 0.25 = 1.75 MAcc inflected. Of course, I'm not sure if the inflection point affects MAcc or just MAtk, so if it doesn't it's always 3.5, but it doesn't matter since the horns always give 4.5 MAcc.
You can also eat
Tavnazian Tacos while wearing the horns... which (among other things) gives you CHR+4. Therefore, with horns, you get +5 skill
and + 4 CHR whereas with the crown, you only get CHR+7.
Horns = Win
Sorry they're not as fashionable.