Hi, yes, this forum was helpful. It confirmed my suspicion that the contractors screwed up, or more likely, were just out to make a quick buck.
On the 6th service call, they replaced the thermostat and drained the boiler again. Somehow that stopped the worst of the banging but it's still not completely gone. I know the piping is the problem and tried calling Weil-McLain, as someone here suggested. Sent good pics but the techs said they didn't see anything wrong with the install. Then I realized they make their money from contractors, so..
Good that you got the same contractor to fix the problem. These guys weren't going to fix the piping and I don't trust them one bit anyway. I work at a university with lots of old steam-heated buildings so I scouted out the steam guy and luckily he does some side work. He sounds like he knows his stuff and was willing to explain it. But he's hasn't come back yet so I've got my fingers crossed.
Thanks for your message.