I think you are. If you say you've snapped the bolt just under the nut then the shaft is just too short to get another nut on. The rivet nut has a normal M6 thread running all the way through it which extends below the nut by almost a cm, this should give you enough extra reach to grab onto the existing broken shaft.
As you've got to push this extended part of the rivet nut though the hole in the header clamps it might be a bit tight, the other post said a slightly larger hole (9mm) was needed to give the rivet nut space to rotate.
Of course all of this depends on how much of the snapped shaft you've still got left sticking out the head, the more the better. You say it snapped right below the nut so this solution should be perfect, no extractor, no drilling out and retapping etc.
Here's a little drawing:-
[smg id=1807 type=preview align=center caption="Capture"]
As you've got to push this extended part of the rivet nut though the hole in the header clamps it might be a bit tight, the other post said a slightly larger hole (9mm) was needed to give the rivet nut space to rotate.
Of course all of this depends on how much of the snapped shaft you've still got left sticking out the head, the more the better. You say it snapped right below the nut so this solution should be perfect, no extractor, no drilling out and retapping etc.
Here's a little drawing:-
[smg id=1807 type=preview align=center caption="Capture"]
Eagles may soar, but weasels don't get sucked into jet engines...