About that third step:
If I right-click the HUSB line, a context-sensitive menu appears. In it, the closest thing to "add a personalized tag" is Add property via the assistant. I clicked that, then clicked "User-defined property..." then typed "_VALID Y" in that line. Did the same for WIFE.
I then ran the verification tool again. The marriage age difference warning is still there, for both the male and the female individuals.
If I look in Add property via the assistant again (after right-click on HUSB), then Available predefined properties is not empty anymore like it was before, but it contains "_VALID (_VALID)" in the left-hand panel, and "_VALID: Custom tag not defined in GEDCOM" in the right-hand panel.
I must be doing something wrong. Maybe it is significant that the warning in the compliance report appears for the two individual entities (I37 and I75), but not for the family entity (F23)?