[Dexter]

The nut and rubber can't be seen looking down drain it has to be seen from under side. The water line to the sink is not a likely candidate as a line usually only fails at a junction or connection. If the particle board as gotten soft even a small amount it really needs replacement. If you stop the leak maybe the particle board will dry up and get hard again, but it usually swells and gets brittle. In any event You need to see under the carpet or tile. Its not unusual to have to make a hole to get to plumbing fixtures. ( The shower Mixer valve in your home prime example) I still believe the leak is only when the water is on. And you are correct that a small leak would be soaked up before dripping. You could try letting the water run in the shower for double or triple the usual time, I don't recommend this because the water may spread the damaged floor. I think you just need to go ahead and take up the flooring and anything else that's is in the way. I think you will find the job much easier than you think once you see it all removed. The way to put it back will be obvious