[wazzz]

OK so now we know your ECU is OK (since you tried both ECUs and got identical results with the known good ECU from your own car).
We now know your flywheel sensors are OK too, since the engine caught when you tried to start it with ether. We also know or "suppose" your injectors are OK, since they have been professionally serviced recently.
The wiring between the ECU and the injectors is indeed a suspect and should be tested. Seeing 12V on both injector connector terminals is normal when ignition is ON. But that doesn't mean that the signal line (pulsed ground) between ECU and injector loom is good, because the 12V voltage can be seen "through" the remaining five injectors themselves, as they are pretty low impedance and show almost no DC resistance being connected in parallel.

As suggested above by Ingo I would disconnect all six injectors from the loom, disconnect the big connector from the ECU under driver seat and ohm between this connector pin 14 and each injector connector on cylinders 1, 2, and 3. Do the same between connector pin 15 and each injector connector on cylinders 4, 5, and 6. For one of the pins of each injector connector your ohm test should show good continuity. Less than one ohm, not taking into account the test leads that must be pretty long in this case since you must ohm between engine compartment and cabin.

Another easier test that doesn't require extended test leads would be to leave all six injectors connected in the engine bay and simply work at the ECU big connector under the seat, of course disconnected from the ECU. Don't switch the ignition on. Now from the ECU connector, harness side, you have access to both injector loops. That is, you can ohm test between pin 14 and pin 18 (pin 18 is battery power, but 12V is not present here as ignition is off, and this pin is permanently tied to the +12 leg of injector circuit). Do the same between pin 15 and pin 18.
In both cases you are measuring the total resistance for 3 injectors connected in parallel, plus the line resistance for these 3 injectors. That should roughly be equivalent to one ohm or less, excluding the ohmmeter test leads.

If these ohm tests all pass, then there is this rare but real possibility that something is wrong with the six injectors together, that makes the ECU injector driver stage "flatline" because of a too low resulting inductance. That has already been shown in past threads where the engine wouldn't start on six injectors but would run on 5, and reconnecting the sixth injector would make it die again.

Also, a shorted injector might prevent the other five to receive any signal, since the ECU will also flatline in this case. You may want to try disconnecting them one by one and see if the engine starts on the other five. Event disconnect two at a time and crank to see if the engine starts on the other four.

Lastly, I can't remember if you have tested your CHT, but that can also prevent the engine from cold starting if it is shorted.