I don't think you need to worry about the oil pump chain, but you should be able to replace it through the sub pan.
The solenoid sounds like the overboost protection switchover valve. Its job is to cut the boost signal coming from the intake and going to the injection pump aneroid if the boost hits some value much higher than the waste gate pressure. I have seen many of the sensor switches go bad causing this to energize. A distinct loss of power occurs in such an event. Unless its just an accident I imagine someone disconnected it for such a reason. The chances are much greater that the system will fail than the boost go too high. The test is to tee in a pressure gauge into the line going to the aneroid. You should get about 9-10psi at full boost. If its low (the more common problem due to carboning of various choke points)the car will have no power (this is the number one cause of such things). If it is too high around 18-19 psi then the car will fly until the cutoff takes place and then drop almost like someone shut off the fuel as the power drops and the boost drops the cycle repeats causing a horrible bucking. Power off, power on, power off, power on etc.
Bosch Master, ASE Master, L1
33 years MB technician