You can't argue that the original deal hasn't worked out well for CCFC, it does make you wonder why the club agreed to it. If we believe PWKH then they took this deal in preference over other options that would have seen them paying less in the lower leagues. The only reason I can think of is that the owners at the time made a strange assumption that we'd shortly be back in the top flight and the rent would be, to a certain extent, insignificant.
If at day one the club had kicked up a fuss you would think they would have had the whole fan base behind them pressuring the council.
Any new owner has a clean slate to pressure the council. First off purchase the 50% stake off ACL and then make an offer for the councils 50%. If the council refuse, and its a fair offer, go public. I think you would then see a great deal of pressure on the council.
If I was SISU I would be making it clear why stadium ownership is needed and how it benefits the club. They've not really done that other than just stating it as fact. It leaves them open to an easy counter argument that the rent only made up a fraction of the annual losses. If they could demonstrate the need for ownership it would certainly change my opinion and I'm sure that of other fans, at the moment I see no reason they need ownership.