Since the Interloper has May 12 as the expiration date, I'm pretty sure that Telltale will honor the deal until then even though the store page states another date.
Not sure I understand your other question, however.
The secret of Monkey Island SE is already available separately for $9.95 and I'm pretty sure that $9.95 is not a promotional but its regular price.
Even though I already have it on Steam (which I currently run on my Mac via Crossover Games), I'm considering buying the game from Telltale so I can run it independently from Steam (in either CrossOver Games, Parallels or Bootcamp... too bad they don't sell it on Mac; I guess I'll have to get the Mac version from
GameAgent!), especially as it's half price if you buy it in the same order as Puzzle Agent.