Well, I have to give the local Mr. Rooter franchise an "A."

And it was a lead pipe but I didn't get any kind of "ZOMG I am so not touching that and you need to evacuate immediately" response.

The problem is almost certainly fixed - I say almost certainly because they said they thought it was best for the sealant to set up overnight (they also had to seal a spot on the sink that was leaking, where the overflow part connects into the main drain). They also put some kind of "enzymes on steroids" (well, that's how the guy described it) down the drain to eat through the clog that they apparently thought was still there.

(The sink has ALWAYS drained slowly; perhaps many years' buildup of soap-scum - from before my time even - finally plugged the thing).

They want me to wait until tomorrow morning to try the sink, and then call them to let them know. They said if it didn't drain right, they'd go up on the roof (!) and snake down the standpipe. (Though I doubt that will be necessary; both the tub and toilet drain just fine).

The manager (the plumber called him out for advice; the plumber was a young guy who had apparently never seen a lead drain pipe before) told me it was highly unlikely they would have to open up the wall, and even if they did, they had a cover they could put on afterwards. And he said that the pipes in the walls were a lot sturdier than the crumbly pipe that was sticking out of the wall (in fact, they'd "probably outlast the house" so hopefully my visions of multiple walls being hacked into won't come true.) (That was another of my concerns, that there'd be a gaping whole left in the wall and I'd have to trek out to Lowe's to get a piece of board cut, and then paint it myself, and then drill it and insert molly bolts or whatever to hold it into the rather thin back panel that makes up that wall).

If nothing more needs to be done, the cost will come to about $150, which is cheaper than I expected.

Oh, and the plumber:

called to let me know he was on his way
explained each thing he was doing
showed me a price list so I knew in advance what it cost
went and got the appropriate parts from Lowe's when it turned out their warehouse was out of them
cleaned up the area
called in help when he needed it, instead of blundering ahead and maybe screwing something up worse.

The manager also assured me that they didn't want me to pay them until they were completely done and I was satisfied that the sink was draining right.

I guess what I'll have to do - as I'm not inclined to give up using bar soap - is remember to put a good dose of boiling water down the sink once a month to try to flush out the buildup before it gets too big. (or see if there's a commercially available version of that enzyme stuff)

So anyway. I'm all for supporting local businessmen but it's dang nice to have a plumber who doesn't suggest an appointment window "between 8 and 5, Tuesday, Wednesday, or Thursday" and who doesn't just blunder ahead and then go "oh, crud" when something breaks because of his blundering.