A resignation doesn't help us. A resignation is usually from a matter of principle, so those who are corrupted can simply sell it as such, disgusted with the process of City avoiding major sanction.
A resignation also doesn't need to be held off for a month, as I can't see City agreeing to let Uefa dress it up as a resignation, rather than a raft of sackings.
Either way, it serves City no purpose, as the mud still sticks with resignations, rather than public sackings.
I think Castles is just trying to wring the dregs of a story which passed him by these last 24 hours.