I had my headlight unit out last week as the adjuster ball had come out of the socket, and the rubber seal had dropped back into the nose. Having not approached this job before, I ended up going the long way round, I started trying to drop the airbox to get access to the front headlight fixing. I soon forgot that plan and went in through grille, much easier.
Anyways, got the unit out, found the ball went back into the socket and held it securely enough.... (meh... will need a to replace said ball at sometime) cleaned up the rubber gasket and stuck it to the headlight unit with some high integrity silicone gasket material, this made refitting easyas the rubber stayed exactly where it should.
Back to splitting the unit, reading elsewhere on other makes, either local heat, or low bake may soften the glue allowing it to be split Loz. I await someone with experience with of this particular headlamp unit to come forward.