The other scenario to consider here is that the Sox would threaten to go after some of Lackey's contract money for prior years if he refuses to honor the option. They would argue, correctly I believe most lawyers around here would agree, that the consideration for the the current contract included his playing the option year at the contractual price if the condition precedent was satisfied (which it was), and if he walks away from the option and tries to play in MLB again that he'd owe them some of the value of the initial contract back. He certainly can retire at any time, which no one seems to be disputing...it's trying to play again where the issue arises, I imagine.
Larry Lucchino, lest we forget, was a world class litigator once upon a time...he's not going to miss an angle here. I agree with the many who have said that this will all be negotiated away (it's really in neither parties interest to go nuclear here) but I think both also have weapons to deploy in the negotiation, even if we never hear about all of them being used. The Sox argument would be that they wouldn't have signed the deal without that option, and the negotiation likely is at worst ambiguous on this point, and thus they'll argue that Lackey's choice is roughly to pay back 1/5 of the contract (which can be debated, but the most likely way it would fall is that the elbow injury was valued at 1 season by both sides, thus the minimum salary option), play for $500k in 2015, or retire permanently.
My guess? If Lackey is good the rest of the way the Sox sign him to something like a 2/$18m or 3/$30 mil extension this offseason that reflects a material discount, but also a big raise on Lackey's alternative scenario.