You need to check the numbers again. Cutting Hartline next season saves the Dolphins over 7M.
http://www.spotrac.com/nfl/miami-dolphins/brian-hartline/. Zero dead money after this season. His deal was for only 12.5M guaranteed.
As for all the other stuff you said, I'm not sure how that applies. If Robinson is a better #2, Hartline will play the slot. He's better at it than Robinson would be anyway IMO. Most likely, they'd just cut him though as they will need to money to make a few key resignings.
Edit: actually spotrac looks to be wrong here, should have run the numbers myself. Even still, it would be worth cutting him as he isn't worth 7+M and they'll need the money. 3M or so isn't much for one year but when you factor in the 7+M for two more years, he isn't worth it.