Thank you LACAllen. I think this answers my question, although I'm not sure which question your "No" is in reply to. Though you didn't specifically say so, I think the phone would still be tied to my iCloud account even if the passcode is removed and the data erased. I.e., as long as the phone is in lost mode and the activation lock is in place, it's still in my account and the phone is useless to anyone who finds it.
I think this is no longer a hypothetical situation. I left my husband's phone number on the Lost My iPhone screen and he suddenly started getting suspicious looking texts from "Apple Support" saying my phone was found at such and such address. All the other location info to date has been sent to my email, not the contact phone number, and it has been from Lost My iPhone, not Apple Support. The last text had the Apple logo in it and asked me to sign in with my Apple ID. So it seems someone has already broken thru the passcode and now needs to sign in to iCloud in order to use the phone.