Yes, you can request a bed rail and it will fit with the murphy bed.
There are lots of rooms within fairly close walking distance to the main buildings, but of course you are not guaranteed to get one in the same way that you are with Preferred Location. The next best categories to ensure this would be Royal Pool View (as all of those rooms are located near to a footbridge over to the main building) and regular River View (which means your room will overlook the pathway that goes around Ol' Man Island, which again gives you fairly good access). Other than that, you can always place a requests, but they are not guaranteed. Personally, I like regular River View as that gives the best combination of a lovely outlook and the likelihood of a convenient location.
If you book King Bed by the way, you could genuinely request one in an otherwise 'preferred' building (14, 15, 18 or 27) or a room with a river view (both are valid options for King Bed rooms) but again that does become a request, and you might be located further away if they can't fulfil that request. Booking King Bed would also be OK in terms of room occupancy for two adults, one child, and an under-three in a pack'n'play, and you would be automatically assigned to the Alligator Bayou section as you would need a murphy bed. A pack'n'play would fit OK into a two-queen bed room too though.
Andre