The Sierra is actually a Black Hawk airframe with Seahawk running gear - unlikely there.
The Navy does not 'own' six MRHs, it draws six MRHs from the ADF's pool of 47 machines, and swaps them out when one comes up for maintenance. Not a likely proposition either.
It hasn't been done on a VUAS before that I'm aware of, but Northrop Grumman is looking to trial fit a miniaturised version of the Triton's surface search radar to the MQ-8C FireScout (Bell 407) - it wouldn't be a huge leap to fit an air capable radar - the back ends and LOS datalinks would be similar, although a suitable aperture/antenna would need to be found.
Perhaps CEA has a solution?
Re Chinooks, there was talk of retaining a few of our D models when the Fs finally arrive, although that depends on how knackered they are from their Afghan deployments. Word is they were fairly sparingly used compared to other forces, so may be good for a few years as training machines or for domestic/PNG/regional assistance type duties.
But I'd suggest aircrews rather than airframes is going to be the limfac on Army Chinook numbers anyway...