If the brown things on the bottom left hand corner of the picture are where you are indicating, then you won't need 'hangers' if you have plenty of rockwool there already.
Covering with fabric is best for absorbtion (ie: making the music in the room sound better) Don't put plasterboard over the rockwool.
This advice assumes that the outside walls of the room (the grey perimeter) is giving you enough soundproofing for your needs.
The grey perimerter is is made by: 200 mm concrete block sand filled rendered one side + 30 mm air gap + 40 mm 40kg/m3 rockwool and 2 plies of 15 mm sheet rock. I think it's so enough soundproofing.
Covering with fabric is best for absorbtion (ie: making the music in the room sound better) Don't put plasterboard over the rockwool.
Is it? If you have the speakers built in to? Im just curious 'cause i remember reading many places in here to make a sealed space for the rockwool and the speaker mount... Please correct me if im wrong..
For wall frames in this case (sand-filled block, air/insulation, frame with drywall) the frames should be either free-standing or at least spaced from the block wall with neoprene isolated sway brackets. The idea is to keep vibrations from one mass from affecting the OTHER mass, this improves isolation a LOT.
For the front of a CR - if you are NOT doing 5.1 surround (or 6.1, etc) then it's arguably better to use solid panels where the speakers sit; one thing that works well is to leave holes in these panels near the floor and ceiling, so the rockwool behind can be a ported bass trap. Soffits themselves are a big project to get right, there are links to Genelec's site for some help on this.
If you ARE planning on surround work, then typically ALL wall surfaces would be made absorptive and any needed ambience added back into the mix with digital reverb.