Interesting. In other words, a large magical ritual might be able to accomplish more with a low man level than a single magician?
That is the entire purpose of rituals. If we go with the idea that magic is like water, and the magician is the nozzle on the hose, then it makes sense that fixing four, ten, or a hundred 'streams' onto a single goal working as one will do better than a single high-powered stream, especially for large-scale uses. When there's a fire, does the fire department send a single truck with a water cannon, or several trucks with hoses that can be used together to combat the blaze? The thing about rituals, however, is that they scale with the power of the individuals involved. For instance, when Great Dragons decide they need to use ritual magic and multiple great dragons to do it, the effects can be world-shaking. Which is why you don't see Great Dragons use ritual magic together that often (also, they don't get along that well).
IIRC Thais was trying to make a spike in the mana level, so it would be easier for other horrors to enter the 6th world way before it would naturally happen. The Great Ghost Dance was one the the ways to make mana spikes. Dunkelzahn managed to stop that, and things should be back to normal now.
That would be... extremely wrong. I don't recall if it went into Thais's motivations or not in Worlds Without End, but it was certainly not bringing back the Horrors. Mana spikes creating an opening for the Horrors was something that was only discovered after the events of Harlequin's Back (Harlequin sending runners to fight in the metaplanes at the site of the bridge), House of the Sun (Harlequin and Dirk Montgomery stopping crazy Hawaiians from doing their own Ghost Dance and letting in Horrors), and Worlds Without End (when Aina hears what Harlequin has been up to, and puts the pieces together with the bad dreams she's been having, and tells all the immortals, including Big D). This threat was ended (for the moment) when Big D blew himself up after becoming President, and the events of the Dragonheart Trilogy (Big D standing all Gandalf on the Bridge, going "You shall not pass!").