The necessity for a split-mould seems pretty obvious - if you stood at the front of the car looking down the middle of the roof, the rebates for the door and the weather channel seem to undercut quite distinctly (just picture trying to lift that mould off the roof and door rebates!) which means you would need to either split the whole mould down the middle of the roof-line or have bolt-on/removable barriers at the edges that form the rebate and weather channel. 

Looking at the detail of the pressed steel around the door closure area and the weather tracks I would have thought that you might even need to do some smoothing out and simplifying of that detail too.