When installing a Murphy-type bed, be sure to install the appropriate framing around the hinged panel, to maintain the strength of the wall. Some Murphy beds in large size have metal portal frames to facilitate this.
The two forms of counterweighting I’d look into would be garage-door springs and cams on “sash weights” – either of which would give you proportional closing effort without either hard pushing at the top or tendency to slam down at the bottom. The idea of having something with ‘hard’ post support with the layout down is a good one, especially if you’re doing the ‘credenza’ with fixed trackage that has to coordinate with the flap when down.