for a loss of 100 psi in a cylinder the things that come to mind are head gasket failed, badly seated valve / burnt ( unlikely , but hey ) and piston ring failure and / or ring land failure (these both happened to mine on no' 2). to be honest if comp's are low in a cylinder the engine will have to come to bits to find cause, however if a single spark plug is glazed black this is oil and it is of my opinion the piston or rings are the cause. if all plugs are of equal appearance there may be a stuck open injector which can wash the bore and cause lack of compresion ( i haven't read the whole post so if not already done swap injector to different cylinder and re check for missfire)
A loss of 100 psi of compresion will cause a missfire as >>>100% good cylinder =150psi so 50psi is only equal to 33.3% , and when the turbo works !!!!! forgot to say just simply remove the rocker cover and check the arms