That's all fine and good except you're missing why the ETF-51D-5NA was even considered in the first place.
B-29B-BO Superfortresses were being lost to Japanese interceptors on the raids over the Home Islands and they didn't have escorts. To solve this problem, they would need... escorts... Yet, there weren't any bases close enough for the Army Air Forces to launch escorts making it necessary to use a naval fighter, which there weren't any of with adequate range. It was also wartime so designing and implementing a totally new fighter in a short amount of time was highly unlikely. Thus, tests with a very simpe navalized variant of the P-51D-5NA, redesignated the ETF-51D-5NA, utilizing re-pressurized tires, a catapult hook, an arrestor cable, shock absorbers, a dorsal fin, a newer canopy, and a slightly reinforced airframe began under Project Seahorse and proved successful. There was the issue of Navy preference of radial engines and the dislike of the fact that there was only an 8mph gap between the stall speed of the Seahorse and the breaking point of the deck cable. The test pilot even thought that the Seahorse didn't have its place in Naval opperations. However, necessity takes presidence over preference and the ETF-51D-5NA would have most likely been produced except for two changes in the war. Iwo Jima and Okinawa were captured by American forces giving the Army Air Force permanent airfields close enough to oppearte standard Mustangs for the required escort opperations. It is for the alternative scenario that Okinawa and Iwo Jima hadn't fallen that the ETF-51D-5NA is being made by Lt. Wolf's team using the model provided by American.
Oh, and in case there are any other critics out there, please allow me to dispel a couple of dislikes of the Seahorse. On the speed gap, the F4U Corsair actually had to be stalled out to land on the deck without breaking the cables, so yes, with the Seahorse, it's bad, but not that bad. Yes, the Seahorse could make the carrier takeoff without the catapult using proper flap and speed settings, but it also handeled well in the catapult. The visibility of the Seahorse was superior to other existing Navy fighter types such as the F4U and F6F. For those of you who argue that the Seahorse wasn't better than the Corsair or the Hellcat in performance, keep in mind that the Seahorse was supposed to escort bombers over very long hauls, not serve as a fleet defender. Personally, I anxiously await Lt. Wolf's Sea 'Stang.
Now, just for Lt. Wolf, I found a schematic of the NA-133 which would have most likely been designated the ETF-51H-1NA had it gone operational since it was based off the early P-51H-1NA, hence the short tail. I thought I'd share since you seemed to be interested in navalizing the "H" once you get the standard done. If you need anything else on this or any of the other Mustangs, please let me know. Thanks...