I think I have (just barely) enough backfill to cover the continuity problem raised by the Willow/Thousands-of-Slayers spell.
Why aren’t they all out killing monsters left and right? Because the only thing about them that changed was their abilities, not necessarily their motivation. They are striving towards the same things they were before: student council, cheerleading, college applications, Girl Scouts, sports… I imagine one or two have joined their schools’ rugby or (American) football teams, but most of them have probably just become better at what they were doing before (with a marked emphasis on athletic abilities). For those of you who have read Fray, think about how strong of a leader she was before her role was discovered. Also remember the montage of women gaining their powers - one woman used her newfound powers to walk out on an abusive husband… but what on earth would compel her to then go off and dust vamps?
The new Slayers are out there, but I doubt they’re all slaying. That’s why the Slayerettes and Scoobies (and Andrew, who isn’t worthy of either title) are out recruiting them!
Why didn’t Wolfram & Hart know about the spell? If it was powerful white magic, and there were no witnesses who worked for the Firm, I can see the wizards at W&H being more inclined to pay more attention to The First (and its potential to cause Apocalypse, W&H’s “mission”) and the Hellmouth than to any spells being cast nearby. It’s also possible that the Hellmouth or The First were “jamming” anyone’s ability to spy on Sunnydale. A wizard trying to “listen” for Willow’s spell in Sunnydale at the end of Season 7 (and identify it, and decipher its effects) could be compared to someone listening to a Led Zeppelin concert in a rainstorm, and trying to make out the words to “Immigrant Song” from a mile away.