The answer is "various times".
Some of the tracks are sketches from throughout the recording sessions, and one, I think it was Trying To Throw Your Arms...well it was little more than an instrumental and Bono's vocals were apparantly recorded recently. You can see the mention of "Declan Gaffney" in the recording credits...he wasn't around in 1991 working with U2...
"Until the End of the World", "Who's Going to Ride Your Wild Horses", "Acrobat" and "Even Better than the Real Thing" all took shape in Berlin at Hansa Ton Studios. As well as work on "Salome", "North and South of the River", and some early work on "Love is Blindness" - mostly Larry drumming...Some of these kindergarten versions could come from that work. I'm basing those on the material lifted from the studio that appeared in the 7 disc set from those sessions..."One" and "Mysterious Ways" also developed there but likely after the stolen material was lifted.
"The Fly", "Ultraviolet" and "Zoo Station" were developed after the Hansa sessions, in Dublin.
In July of 1991 they were working to finish off songs, and that included changes to mixes for "Mysterious Ways", "One" and "The Fly". So some of those could be alternate mixes created during that time.
No good answer, but I feel that the Kindergarten disc was not an "alternative" version of the album, but was instead assembled from various demos from various points in time instead...
