If cold start valve is stuck on... all your plugs would foul out.
Disconnecting the cold start wires woudl disable that device.

I believe you are on the wrong track.

The function of the OVP relay is to protect the electronic circuits of
the injection and ABS computers from excess voltage by shunting the
overvoltage by a zener diode, thereby blowing the fuse.

The injection system then goes into failsafe mode, which means it
operates by mechanical means only without computer control.

When you pull out the OVP relay, you force it in failsafe mode, and
apparently this works fine, which means that the mixture (which is
adjusted by the screw inside the tube accessible from the top of the
air filter... do not touch it !) is OK and that the injection system in
general is OK.

In normal mode, the mixture is fine adjusted by the fuel pressure
regulator, which is controlled by the injection computer according to
feedback from the oxygen sensor and inputs from air mass and
temperature sensors.

So, the problem is most likely within this loop.

You already replaced oxygen sensor and fuel pressure regulator, so the
next major point to suspect could be the potentiometer in the air mass
sensor.

You can measure the input to the computer by a multimeter. With igntion
on but without the engine running, the voltage should vary continously
from approx. 0,5V to 4,5V proportional to the position of the air mass
sensor plate.

I had a similar problem with flooding (except I could not start at
all), and in my case I replaced the fuel pressure regulator (EHA) and
thereby ruled this out as the cause of the problem. After trying to
find the problem for months, I decided to replace it again, and whoops
it worked immediately. So don't rule out the items, you have already
replaced.