The good news is, the sink in my mother-in-law's bathroom no longer leaks. Also, we now know that whatever is causing it to drain so slowly has nothing to do with the P-trap. Also, I can now remove and replace a P-trap without causing a sink to leak all over the place when it didn't before. The bad news is, the sink still drains every bit as slowly as it used to; if anything (and I hope this is just my imagination playing mean tricks on me because the alternative makes no sense), it's worse. Also, I wasted two hours of my life learning how to remove and replace a P-trap in a way that doesn't cause the sink to leak all over the place (namely, do it over and over and over and run back and forth to Home Depot and keep tightening shit until it stops leaking why won't it just stop leaking?), but I still don't know the phone number for the plumber, who is the person I am going to call when any plumbing-related issues come up again.
On the one hand, the philosophy of, "oh, that's got to be pretty easy" was what lead me to being fairly proficient with computers these days. It did so by way of, "oh, shit, I had better figure out how to fix that thing I just broke before mom and dad notice." (Mom, dad, that happened way more than you want to think about.) On the other hand, plumbing won't let you play Wolfenstein 3D if you just tweak it a little more, so there's about zero chance that I am going to devote the kind of time to getting good with plumbing that I did with computers.
As it turns out, my time with computers has taught me one valuable thing about plumbing. The next time I notice that my mother-in-law's sink is draining slowly, I will very carefully direct my web browser to www.angieslist.com and find someone who did whatever the equivalent of "kill demon-Hitler" is in plumbing.