I can't be convinced that Willow had the chops to bring Buffy back from the dead - but wasn't able to see where Buffy was and if she was in need of resurrection. That's where and why she was wrong, IMO. And once she was back, they needed a
Slayer - but it really seemed that they wanted to pretend that Buffy (their friend who they loved so much) wasn't at all affected because they had no idea what to do. So they did nothing. They just left her to slay, take care of Dawn, and try to pay for a house they were living in. Xander and Anya both had decent jobs - but they never offered to help.
My point is that it was obvious that Buffy - not the slayer but Buffy - was drowning. And they just ignored that.
If getting Buffy back was the key thing - then why did they all pat her on the head a few times and leave? Willow knew Buffy better than anyone - why didn't she take the time, more than once, to just sit with her and be with her? Because Willow wanted credit and a pat on the head, and she didn't want to see that anything was wrong.
That's where I think they felt better saying they were doing it for Buffy, but having the slayer back was what they really wanted. Willow hid what she was doing because she knew she was wrong. The others hid what they were doing because they doubted but had made a promise. No one even bothered to see if Buffy - the chosen one - was really in a hell dimension. Because they needed the quippy, supportive, slayer back to take care of them.
YMMV - this was not about taking care of Buffy - this was about taking care of them.
Buffy dying didn't call another slayer because at that time the line ran through Faith, not Buffy. It went Buffy-Kendra-Faith. So when Buffy died a second time there would only be one slayer.
But apparently the resurrection spell called back the 'warrior of the people' and 'Buffy the Vampire Slayer' was reborn. The slayer line now ran through her AND Faith. So from that point on there would always been two girls in all the world.
That's why the First got all pissy and took the opportunity to start killing potentials in S7.
You just explained that to me better than I had ever realized! Thank you!