You don't want to make it utterly impossible for a junior diplomat to override a senior one - what if an interesting deal presents itself after the senior diplomat has left for the day, or if the senior diplomat has made a glaring mistake? It should only be possible if the junior diplomat declares that they're overriding the senior one, risking their wrath, in my opinion.
"preliminary orders can, as usual, be overwritten by junior diplomats,
signed,
senior diplomat"