Alist of which npcs and locations sell every spell is found here: http://www.twpa.net/ffxi/spells/ Very useful website. Also remember that the magic shop in whichever nation is in first will sell more spells than the others. I know for sure that this spell is sold at the magic shop in jeuno all the time though.
After raise, your state is "weakness", not curse. So, no, cursna does not remove it. To my knowledge, the only ways to remove "weakness" are time (waiting it out) and killing yourself/respawning at HP.
I have not tried it (as my whm is only 28) but I do beleive that you can use Cursna on the whole party the same way as with Erase. Just Divine Seal to activate Divine Veil(lv 50 ability) See link:(http://ffxi.allakhazam.com/db/spell.html?ffspell=559)
I have this spell, and the target type is completely wrong. It effects one person, just like all the other -na spells. That person does not need to be in your group, and it's single target regardless.
It would be nice if allakhazam worked on putting info about where spells are sold because it would really help us WHMs and BLMs since AH usually has spells for atleast 1000 gils more than normal price.