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But somebody, somewhere, at some point in time, HAD to know that Angel, or at least an ensouled vampire, was "coming," otherwise there would never have been a Shanshu Prophesy, and that someone, probably the ancient witch/sorcerer/demon/whatever, knew about the happiness clause in the spell. Most likely, it was like the spell book in "Doctor Strange," where the consequence of the spell, and maybe a way of eliminating it, was written on the final page of the spell book, and it got torn out and lost millennia ago.
I'd say no. I don't think Ethnic Stereotype laid out the terms to him between his ensoulment and Darla's massacre. As for the spell, who knows? I could see it as the Gypsies having the power to re-call the dead from Hades, and then to bind them to an undead body, and some extremely gifted witch figuring out how to turn that against vampires. The happiness was a glitch, Hades (or his non-union Buffyverse equivalent) was happy to let the dead out for creating revenants, maybe to farm them out for day labor, or let them talk to the living, or to deal with unresolved issues, whatever. The curse was to keep the shades of the damned from becoming happy So perhaps the curse was an off-label use of a spell? Like using "heal" on an undead. The Seers could have foreseen the Vampire with a soul, but how was this done would remain a mystery.