hey good job troubleshooting!
Seems you've narrowed it down to a few possible culprits:
1. power to injector #2. Key ON, measure voltage going to each injector.
2. trigger signal going to injector #2. Use 'noid light to verify each injector is being fired by ECU
3. ECU may be suspect if it's not firing only #2 injector. Test trigger signal at ECU itself. If present at ECU connector but not injector connextor, then wiring in between is suspect.
4. I still suspect spark issue. Test with actual plugs. Insert plug into wire and ground case to engine and crank. What colour is spark? Do test for ALL cylinders
5. do compression-test with warmed up engine.