Under Floor Heating (Wet) - pipe coupler under the screed?
Can i use pipe couplers to connect then all the rooms to the system(manifold in the hallway)? and submerge the couplers under the screed? 


With PEX pipe it depends on the fittings. Some fittings are rated for this (the crimp/slide ones), others are not (ie, the ones you screw on). Check your local code.

Personally though I wouldn't risk it. It would be easier to simply leave enough pipe in the corridor, then roll it and place it out of the way against the wall so no-one trips on it.