After the season he put in last year I wouldn't label Florence as a "serviceable nickel back". Florence should shine playing more man coverage, and he would be solid #2 CB on many teams in the league. I wouldn't call McKelvin a huge bust, at the least he's been a serviceable #2 when given the chance. McKelvin has all the physical tools to succeed, and hopefully with more reps comes less mental breakdowns.
Using comparisons from the same draft as way to make your point is pretty weak too. By that logic Jonathan Stewart is also a bust because he's not Chris Johnson...who was taken later in the draft ala Rodgers Cromartie.
Reggie Corner is also a serviceable nickel, however I don't think he is right for our new D. He has shown enough these past two seasons to warrant being on this team. However, I don't believe our new D will allow him to shine, as he operates better playing off his man in a zone.
Not to mention, your ratings seem like they aren't taking into account where each player sits on the depth chart. Yea, every team would love to have four #1 CBs, but that's not gonna happen. If you were to compare McKelvin to other nickel CBs around the league you would be hard pressed to find 5 who are better.