The newest verified filename is a useful convention, but modification time and acceptance order can diverge when files are copied or a late worker finishes design A after design B was accepted. A later timestamp should not give that old proposal authority again.
I would keep one small current-design pointer containing the accepted revision and require a proposed update to name the revision it started from. A stale proposal can remain as history without replacing the pointer. That adds a concrete acceptance rule to the files-first approach while leaving the heavier memory graph unnecessary for this particular question.