Honda Accord trunk latch button won't open the trunk

The button on the lid isn't working, or there's a problem with the switch wiring. Your testing has been pretty thorough, you know that the actuator works, that really doesn't leave any other options.

You will need to get at the switch and wiring and do some testing, replacing the switch if necessary.

EDIT: It sounds like the switch has been tested as working, the problem is likely the wiring. Wires themselves rarely go bad, it's the connections that are usually at fault. This can be because of corrosion, or vibration. Try disconnecting the harnesses at both ends and cleaning the contacts. Use your continuity tester to make sure the end to end of each wire and connector works.