Another option is that 28 Sqn get rid of their Merlins and they get modified to some (probably poor) SAR role
I can't see that happening any time soon. If, as is widely expected, there is hurrumph hurrumph further restructuring, I wouldn't be too suprised to see 230 Sqn start drawing down in NI as part of the peace process out there. Given that to do so, and if drawing them down involves them being pensioned off rather than re-allocated, would only compound the already dodgy state of the battlefield helicopter fleet identified by the NAO. To then farm 28 Sqn off to SAR duties on top of any reduction in the Puma fleet would end probably cause a lot of arguing at senior levels as more pongos would have to walk to battle rather than getting a cab.
If I was a betting man, I would say that if anything, 28 Sqn's future is fairly secure as it is - as long as it can get all the gremlins sorted an work out their conops. Fewer Pumas on the books means increased likelyhood of Merlins deploying - as good as the Chinook is, there will be times when it is overkill and simply not suited to the requirements of the job.
Think you may have to look elsewhere to find your new SAR cabs. Who know, I wouldn't put it past this bunch of muppets to axe SAR totally as a military function in the UK and farm it off to Bristows or some other company, whilst developing a smaller CSAR fleet that can be taken off on ops - now that may become a 28 Sqn role, but I doubt they'll be buying up cans of yellow paint at Benson in the short term.