Ok, I picked up the car from the mechanic today, so here's some more info:
Since I couldn't get an appointment at the local engine mechanic until later this week, I went to the original mechanic (the one that declined to do anything after seeing "oil in the coolant") to pick up the car. When there, I took a photo of the interior of the coolant surge tank (attached). I added about 2/3rds of a gallon of distilled water to the tank (to a little past the fill line after running the engine for a minute) so I could drive it home.
I also re-checked the oil. When the engine has been running, all three segments of the indicator are filled with normal-looking oil. However, if I wipe the dipstick off and re-insert it with the engine off, it comes back out completely oil-free (not even oily to the touch). Is this normal?
The ~3 mile drive back home was unremarkable, other than that the temperature gauge kept climbing past the middle of the scale (where it usually hangs out), and was at about 60% and still rising when I got home. No smoke was coming out of the exhaust pipe, and no more water/coolant was leaking from the surge tank.
At home, I took the two attached photos of the surge tank.
Then I opened up the intercooler pump fill nozzle, and tested to see if anything was flowing by manually squeezing the hose to the left of the nozzle. I didn't see any turbulence.