There was nothing in the Seafire that entered service in 1942 that was not reasonably feasible in 1939, albeit with a less powerful and differently blown Merlin.
The 1939 time line is rather iffy.
Let's assume that you have enough constant speed props as a given or the the idea won't work to begin with.
you have an 880hp engine for take off with 87 octane fuel. In Jan of 1939 you know that 100 octane fuel is coming, you don't know when or exactly how much improvement you are going to get. As the months go by you are able to start planning the arrival of 100 octane and which airplanes are going to get it. You also start to get an
idea of what the performance gains are going to be.
Yes the Merlin 45 used in the First Seafires used a different blower but it had 1185hp for take off, without using emergency power.
In 1939 (?) there was proposed version of the Merlin to use 100 octane that was rated at 1000hp for take-off using 8 1/4lbs boost. As the results came in from testing the existing Merlin IIIs with 100 octane fuel they found they could simply upgrade/up rate existing engines and a new version wasn't needed. But when did they know that?
There was an early proposed Merlin of the R.M.1 M type with a lower ratio on the supercharger to use 87 octane fuel that would give 1000hp for take-off using 6lns boost, but it would only give 1085hp at 9750ft and then decrease after that.
The engine (VIII) in the Fulmar I used an even lower supercharger gear ratio and was good for 1080hp for take-off at 5lbs boost on 87 octane fuel and about 1275hp at sea level using 9lbs of boost. This was a R.M 3 engine.
The R.M. 3S engine was the one used in the Spitfire MK II and was rated at 1175hp for take-off using 100 octane and 12lbs boost.
You may not need the Merlin 45 but the timing for the Seafire gets a bit tricky in 1939-40.
Skip the Fulmar, threaten the future Sir Charles Fairey with nationalization of his firm if he persists with his Fulmar folly
All these threats against British manufactures wouldn't have changed much.
Just tell them what you want them to make and pay them for it. Just don't change your mind unless you are willing to pay for the raw materials, subcontracted parts and labor for the partially built stuff they have on hand when the air ministry changes their minds.
Remember the Hurricane story. Hawker went out on a limb and ordered raw materials and long lead items
before the Air Ministry actually placed the order. If the Air Ministry had changed their minds and only ordered a hundred of so Hurricanes while they waited for BP Defiants (or whatever the flavor of the day for turret fighters was) Hawker might have faced bankruptcy.
Fairey and company would pretty much make anything the Air Ministry told them to make if they thought they could make money doing it.
Fairey got an order for 127 Fulmars in mid 1938. It took until Jan 1940 for the first one to fly.
When do you cancel all the parts and tooling already in process of making the Fulmars and how long is it going to take to set up the Seafire production line.
Somebody can "say" they had the wing fold all worked out and ready to go but let's face facts. Supermarine (Vickers) was making Spitfires in very small batches or constantly changing things even as they were building the first hundreds. Something that drove Castle Bromwich nuts. Granted their management wasn't the best but the constant flow of change drawings for minor items would have been a problem for any 3rd production line. It is either that or the Seafire as building in late 1939-40 becomes increasing deforced from the Spitfire production standard. A lot less interchangeability with the land based aircraft. How important this is?????
Look at Eastern Aircraft and the Wildcat. There were literally hundreds of drawings to change from the 4 gun to the six gun wing.
Do you want Fairey to make "Seafires" or do you want them to make a single seat carrier plane that just sort of looks like a Spitfire from medium distance?
And we haven't even gotten into the A wing with eight machine guns or the B wing, which one do you figure out how to fold up in 1939? You know the 20mm guns are coming but they don't show up in any real numbers all the way through 1940.
And what do you do for scout aircraft, the often ignored role the Fulmar played in chase of the Bismarck.
In the Med in 1940 an early Seafire will outperform anything the Italians have got. It better as the early Seafire is going to have to land twice as often to refuel and rearm.
There were reasons the ammo capacity and fuel capacity was spelled out the way it was. It may have been wrong (lord knows the carrier turret fighter was a bad idea) but a 1939
Seafire with eight .303 guns with 300rpg and 85 gallons of fuel was not be a total replacement for the needed fighters.