@ Schwarzenberg:
> That time seems off. Are you sure he didn't mean 5:26?
The 6:26 time is what I found in Churchill's book. There might be a typo in the book, and I also wondered about a difference in time zones. But Churchill says that the fighting had already started when the message was sent, so it can't be 5:26.

This is the relevant part of the account in Churchill's book, copied verbatim. (and with particular care to the times - btw, you'll notice that the last time quoted is 6.0 P.M., which I take to be a typo for 6:00 P.M.).

All day negotiations continued, Captain Holland waiting in his
destroyer off the boom. The distress of the British Admiral and
his principal officers was evident to us from the signals which
had passed. Nothing but the most direct orders compelled them
to open fire on those who had been so lately their comrades.
At the Admiralty also there was manifest emotion. But there was
no weakening in the resolve of the War Cabinet. I sat all the
afternoon in the Cabinet Room in frequent contact with my
principal colleagues and the First Lord and First Sea Lord.
A final signal was despatched at 6.26 P.M.
"French ships must comply with our terms or sink
themselves or be sunk by you before dark."
But the action had already begun. At 5.54 Admiral
Somerville opened fire upon this powerful French fleet, which
was also protected by its shore batteries. At 6.0 P.M. he
reported that he was heavily engaged.

> the ultimatum had technically expired at 3:00
I think this might be a further discrepancy between Churchill and your "source" (the JMH article?).
Churchill does not quote explicitly the expiration time of the ultimatum; however, he reports the full text of the ultimatum, which includes the following lines:

If you refuse these fair offers, I must, with profound regret,
require you to sink your ships within six hours.

[I should note that these lines clearly say that scuttling by the French was considered an acceptable (if not optimal) solution.]

Furthermore, Churchill also says that

The Admiral [..] was off Oran at about 9.30. He sent Captain
Holland [..] to wait upon French Admiral Gensoul.

if so, the ultimatum was delivered at 9:30 or later; and it should have expired after 15:30 (or even later, if Somerville/Churchill were willing to account for the delay in reaching Gensoul).


> Also I had never heard of this plane being shot down.
This is from wikipedia, not from Churchill. It says
Before negotiations were formally terminated, British Fairey
Swordfish planes escorted by obsolete Blackburn Skuas were
dispatched from the Ark Royal to drop magnetic mines in the
path of the French ships' route to sea. This force was
intercepted by French Curtiss H-75 fighters. One of the Skuas
was shot down by French fighters and crashed into the sea,
killing its two-man crew, the only British fatalities in the action.
wikipedia also gives a source about that,
Christian-Jacques Ehrengardt & Christopher J. Shores,
L'aviation de Vichy au combat. Tome I: les campagnes
oubliées, Lavauzelle, 1985