The stockholders would likely have to sue the Board as the board can always reject an offer as too low.
Plus there were all those powers they granted themselves some months ago.
It's all very cloudy in that respect. I also think they may have some sort of informal promises to MRK in place, along the lines of ... we'll make a deal on Rida, if that one doesn't quite work or especially if it does, the price will be going up, then you offer us $XX agreed upon, and MRK buys it.
Hard to say.