So the P-51 (Allison) was in production in 1941 and the MerlinXX was in production in 1941, both in the USA, but there is no way they could have been used together? To get a fighter in production before the P-38 got into combat in very late 1942?
Ah.........no.
Please note that as of Jan 1 1942 there had been 207 P-38s built for the US forces and 138 Mustangs built for the British. Also note that Lockheed was building , at a minimum 100 P-38s a month for most of 1942 (August was an exception) Lockheed built way more P-38s in 1942 than NA built Mustangs (in part because the Mustang had not been ordered in the numbers that that P-38 had been)
Changing the goalposts and comparing apples to oranges doesn't help discussions.
You asked about the initial British order for Mustangs. It was placed 2 1/2 months before the deal with Packard was signed although Packard had been working for several weeks prior to the actual signing. The British ordered an additional 300 Mustangs in Sept of 1940, during the BoB. At this point it seemed that the Hurricane needed the Merlin XX to stay competitive as a front line fighter. This idea went out the window When the 109F showed up. However that is 4-6 months further along. Allison engines are already ordered for the Mustangs, playing switcharoo with the engines at this point is difficult. Sticking Allisons in Hurricanes would be a disaster from a performance point of view.
Now you want to change things to some air force (US?) getting Merlin XX powered planes in
production and/or initial service BEFORE the P-38
sees combat.
These are two different things. And the contention that the P-38 didn't see "combat" until late 1942 is also somewhat mistaken.
April 16th 1942 sees F-4 photo Lightnings flying recon missions over Eastern New Guinea and new Britain (they used 75 gallon drop tanks to get to Australia )
May 29th 1942 sees P-38Es start their deployment to Alaska.
The 1st Fighter Group is playing ping-pong flying back and forth across the Country, initially deployed from their base in Michigan (Selfridge field) to the west coast after Pearl Harbor they headed east in April to get ready for deployment to England, They got their older P-38s replaced by P-38Fs. June sees them heading West again because of Midway. They are turned around before they get there and go back to Maine (jumping off point for the transatlantic flight). Over the Summer two P-38 fighter groups (six squadrons ?) cross the Atlantic by air. Early Sept sees a number of sorties over France as the groups work up but with no contacts (combat?) Oct 15th sees the first B-17 escort mission. Oct 31st sees all P-38s in England withdrawn from operations for use in Operation Torch.
In the Pacific the 67th fighter squadron with a mix of P-38s and P-39s arrives at Henderson field On Aug 22nd. Oct 31st sees the 339th squadron with P-38s arrive at Henderson field. Perhaps you define that as
very late 1942?
Now
perhaps our mythical Merlin XX Mustang could have been built by the hundreds in spring/summer of 1942 (and the P-40Fs built as additional P-40Es?)
But they would not have been able to fly the Atlantic. Would they have been able to fly from England to North Africa once the ground troops took a few airfields like the P-38s did?
The British Mustangs flew their first operational mission on May 10th 1942, shooting up a German Airfield just across the Channel. As stated before the British have 4 operational squadrons in the middle of Aug 1942 at which point both NA and RR are working on building prototypes of Mustangs with two stage engines.
There is often a 4 to 6 month delay between a fighter going into production and it seeing squadron use (in it's first squadron let alone large numbers) with combat often taking several months more.
There is alo the need to allocate resources (like engines) so they get the highest number of
effective aircraft rather than a lesser number of great fighters.
This was the whole Spitfire vs Hurricane II debate. The Hurricane II got the better engine because they felt that without it the Hurricane ( being built in larger numbers than the Spitfire) would be obsolete without it and the Spitfire (in the MK II version) was good enough in the interim.
Would a few hundred Merlin XX Mustangs (or even 500) made up for not having the P-40F? Would the Merlin XX Mustangs have been sent to NA to provide top cover/support for the P-40E/Ks and not used to escort B-17s in any case?
Yes a Merlin XX Mustang would have been better than the P-40F but you can't have them in unlimited quantities and something has to take the place of the P-40Fs and Ls or the Allison P-40s despite how well they might have done at low altitude are going to get pounced on from above more often.