While you can create spiracles that repel polar substances like water, you cannot protect them against surfactant like soap. No more showers unless you want to drown.
You can waterboard them with it. If you want to effectively kill them, I'd recommend using the tried and true neurotoxin option sold at your local grocery store. You can be your own Saddam Hussein without having to hide in the most random places after committing your war crimes.
Not necessarily. I'm far from sold on the idea, but lungs are able to take in enough oxygen to hyperventilate easily, clothes would reduce the maximum air intake to some extent, but from l full capacity would be max you need to breathe to max out your muscles - everything past that would be excess capacity
If we're going for redundancy and throughput, clothes probably would be more useful as a filter than being a limiter (and filtration is why I'm not on board with this design)