Jean
The receipt of the instructions from Britain in January 1800, before the Convention of El Arish was signed, clears both Lord Keith and Nelson of refusing to ratify it on their own. They were acting under instructions of their government. Sidney Smith if being unaware of them, of course, negotiated in good faith. Unfortunately for him, he had no authority to ratify the convention on his own.
Sidney Smith was a Captain and Commodore commanding a squadron in the Mediterranean. He was under the command of Rear Admiral Nelson and so under Lord Keith as Commander-in-Chief in the Mediterranean. Smith knew he was under Nelson's command, as in late 1799, Nelson was ordering him to join his command.
The Convention of Cintra was conducted under the orders of the Commander-in-Chief in Portugal and not one of his junior officers. Dalrymple was then in a similar situation to Lord Keith. Dalrymple as CinC could ratify a convention if authorized by his government. Cintra caused a furor in Britain over the fact that the French were not made prisoners of war. The same objection to El Arish. Cintra resulted in a government enquiry and Dalrymple was never employed again.
I think you will find that a number of the conventions for the surrender of the islands in the West Indies were subject to ratification by the Commander-in-Chief of that theatre or authority was granted to those who negotiated the convention. It would also be interesting to see how many conventions resulted in the garrisons being held as prisoners of war?
By the time Menou refused to ratify the convention pending instructions from the Consuls, wasn't Napoleon in charge? Wonder what Napoleon's answer was?
Good hunting.
Ron