You need two level four keyspheres to get full life. or at least two keyspheres, dunno if they're both fours. It's in rikku's grid, and like the first thing inside the lock is a luck+ sphere. (aha, according to my handy map of the sphere grid, it's two fours and a three on one side and three fours on the other)

You weren't playing on the expert sphere grid, were you? or whichever one in Internation that isn't the same?

Behold the Void, yeah, sure you could do it, but it's easier to earn money in zanarkand, and then you get to walk all the way back to the calm lands. usually right after the calm lands people have no gil from yojimbo, or enough for a single bribe. But it's not really all that important, though, really what's important is the insinuation of double casting holy. I'm still thinking that's not possible.