Err, a little help here...?
From what I remember at the time I'd heard It was the WB that wanted Spike over onto AtS or they wouldn't renew it. Whedon never thought of having Spike back, and when he did he only wanted his appearances to be in the form of occasional flashbacks from Angels past scattered down the season.
It was JM who then put his foot down and said he didn't want to go through having to re dye his hair just for a limited appearance as it would make it harder to find jobs elsewhere at the same time. And so in the end Whedon relented and they took him on as a full member of the cast. Was that how it went because another poster on Buffyforums has said to me that...
"No, you're wrong about it.
Ever since S7 started, Whedon told Marsters that Spike would die at the end of the season, but not to worry because he was going to come back. As a matter of fact he told him (paraphrasing) "we have no idea what to do with you this season, all we know is that you'll die at the end but don't worry it won't be for long"."
Is that right? I know they had a loose idea about the whole Faith/Ghost Spike thing, but was that a cert?