Hard starting when warm
I have been chasing the same problem for a while now on my '86 2.3-16V. Often the cause for this is a rapid bleed-down of the fuel pressure after stopping. If you then try to start the car before it cools down enough for the cold start injector to operate, you get the hard starting syndrome.
It looks like you have changed everything that is likely to cause the rapid bleed-down except the cold start valve and the fuel pressure regulator.
Various manuals give tests to determine which element is sick, often requiring a fuel pressure gauge to be inserted into the high pressure fuel lines to measure pressure versus time or operating condition.
The fuel pressure regulator is NOT supposed to "let the fuel return to the tank ...when the car is off". It is supposed to RETAIN most of its pressure. One test for the regulator is to remove the fuel return line to the tank and see if any fuel is released by the FP regulator when you shut down the engine. If there is fuel released, then the FP regulator is bad. Under NORMAL operation it lets a little fuel go back to the tank as it controls operating pressure.
I believe the test for the cold start valve is to remove it from the intake, leaving the fuel line connected, and then see if it leaks down when the engine is stopped.
If you do any of these tests, BE CAREFUL, as the fuel is under upwards of 80 psi pressure!! I would not do them without the guidance of the manuals, to avoid major spillage of fuel onto your hot engine (boom/fire/etc.).