NO_DBL_VOTE does no kill the diagram as shown. NO_DBL_VOTE is activated only when the target (newer) checkpoints are different. As long as the target is the same, votes with different sources are not counted as double voting. The proposed solution does not remedy the situation. The diagram’s links spanning 2 dynasties only involve X whether the source or the target is concerned.
In your modified scenario, since B’s direct right child does not detect any justification link that reaches B, B’s right child and its descendants are not even justified. So your modified scenario does not pose a fork.
Thinking about it, one solution would be to make NO_DBL_VOTE rule stronger again, by banning citing different sources even for the same target. [Now I see it’s formulated like that already. Let me put it differently, until I found the above diagram, I wasn’t sure if NO_DBL_VOTE needs to punish different sources. Now I’m sure NO_DBL_VOTE needs to punish different sources.]