I am never one to use parts to do diagnostics, but this is a special case. You have two control systems down and it occured all at once and they have one thing in common. And that one thing is probably the number one pattern failure on your car.
The OVP relay is something like a surge protector for the expensive controls units: engine management and ABS. The logical test would be to check power supply to both control units. This would not be an easy task. You can guess that the ABS is out if the light is on without driving. You can test the EHA current which is the output of the KE controller (and the reason you are having all the engine performance problems - no fuel control).
The OVP is behind the battery. You must remove a thin plastic shield to see all the control units and the OVP. See if the fuse is blown for sure, but if the date code on the unit is 1987, replace it.
Bosch Master, ASE Master, L1
33 years MB technician