I had just this problem under identical circumstances. I bought some kingspan insulation, cut it to the width between the joists and put it in from underneath. Tape up the joints with duct tape. Its not easy on your back with just 2 feet of headroom but it has cured my problems. Just make sure...
You are ok to paint as soon as plaster is dry. I would suggest that you water the paint down slightly so it soaks into the new plaster. If not you may find that the paint flakes off in time.
I do this regular to the plinths on bars in pubs to form a kick plate. I use evo stick adhesive. Just make sure you follow instructions on the tin and i guarantee you will not have any probs.
You can add sharp sand to self level which does allow you put it down much thicker. If you are tiling the floor it would be ok anyway cos it only cracks if traffic is directly on the screed.