The other points you raise are fair, except I disagree about the costing too much.
I mean, Seattle eats the signing bonus portion, so you have 2 years at $11.4M, $11M. That's not totally unreasonable for one of the top CBs in the game at the tail end of his prime (age 29 and 30 year). It's less than Trumaine Johnson, Janoris Jenkins, Joe Haden, Patrick Peterson, Jimmy Smith, and Aqib Talib, slightly ahead of Darius Slay or Vontae Davis.
Even with Butler's "cheap" year, if we franchised him afterwards, it would still only be $3M cheaper than having Sherman for 2 years.
So I don't think he would cost too much. There are lots of other questions, yes, but that's not one of them.