My daughter's 2001 Civic had similar symptoms which I mistook, maybe like your mechanic, for a leaking oil pan gasket. I changed that out, but it still leaked, with the wet area still on the oil pan.
On one of the Civic websites, there was a discussion where several owners also thought the pan gasket was bad but after changing it, it still leaked oil.
Someone there suggested (doing this from memory, so my apologies) there is an O-ring on an oil line that is the more likely culprit. The O-ring isn't expensive but seemed to be a PITA to correct; a few hours if I recall correctly, and in about the area you suspect. I tried a few high mileage oils to no avail. For O-rings, my understanding is high mileage oils aren't that effective.
Given her car had about the same miles as yours but she was planning to trade it, we agreed buying oil at $3-4/qt. was a lot easier. That said, if you plan to keep the car a while, as a Civic with 170k is about mid-life, you may want to get that fixed just to eliminate the annoyance. The oil loss was not bad, maybe a quart in 2000-2500 miles.
Your diagnosis sounds more like what's going on vs. the oil pan gasket. IDK if you have to remove the oil pan to do the O-ring, but if you do the pan gasket would be replaced anyway. Maybe see what your mechanic wants to do that.