Kelly would not join on 4 years, so I am not sure why you are suggesting it. He has a guaranteed 8 years at GWS, hence Norf with the big offer..
With Cerra you trade SPS and possibly another player plus 2nd rounder. The problem with Cerra is his asking price which Freo are likely to match.
Doesn't matter if they match, he isn't a FA
Nor would I give up SPS in a deal
You don't trade best 22 + extras to obtain another best 22