As a avid post war US auto fan its almost impossible to come close modeling many mass production cars of the time which accounted for a huge chunk of global production at that time. Example being the hugely common 57 Chevy 283 power pack with the 4 barrel or mechanical fuel injection, 4 speed automatic which was installed in several million cars by 1960, the 1955 Chrysler hemi with the dual 4 carbs, etc. Many of the carbs released in the early 50s were used well into the 60s without changes, and the ubber common Rochester 4GC 4 barrel used in multi million cars is more reliable the the quadrajet which ive had to rebuild both on multiple vehicles ive owned.
My point in all of this is when a good percentage of global production had access to this technology why not make it available to a company? Especially when taking into account US manufacturers offered these items to other companies (British, Australian, and french as examples). I think to keep it realistic to access tech before its agreed release date should cost much more research and less a hit on reliability to keep true to historically significant tech. Other items such as mechanical fuel injection always was finicky and unreliable however.
50s? Try even earlier. The first gen Hydramatics came out in 1939 and were so overbuilt that they were used in M3 Stuart tanks in World War II. Granted it was a Cadillac and Oldsmobile exclusive until the 50s but even so Buick adopted the 2-speed Dynaflow post-War as well.
Right now, these are limitations of Sandbox where the unlock years are determined by date of widespread / common use. But in the Campaign, you will be able to invest R&D into tech and get it to unlock earlier, so a lot of these will get at very least partially addressed. The rationale as that you should be invested in playing the long game as well as the short game.
The automatic transmission one though I do agree with because automatic transmissions were very popular in the 1950s and onward American market and 2-speeds are all you can use at first when the very first automatic was a 4-speed.
Mechanical injection isn't strictly speaking pure mechanical. It refers to the control system being mechanical. The injectors themselves are often still electrically actuated even in "mechanical" injection systems. It would probably be better described as "analog" injection and "digital" injection referring to how fuel is metering is controlled via mechanism or computation.
The Bendix "Electrojector" injection system used in Chryslers is pretty one-of-a-kind and you can argue about how it should really be classified. Is it more like EFI or is it more like mechanical injection? Most consider it an EFI system. Its a moot point however because it was NOT reliable as you suggest and most cars fitted with it, of which there were VERY VERY few were later retrofitted with carburetors.
There were numerous other experiments into fuel injection in the late 1950s, Nash and Rambler among them who incidentally also used the Bendix injection system and were no more reliable in those cars so it was dropped in the early 1960s. The only fuel injection system produced by a US manufacturer – that I know if anyways – for any reasonable amount of time before the Cosworth Vega was the Rochester mechanical injection system on 327 C2 Corvettes.
IIRC the Rochester injection was also available in Chevy Tri-Fives at some point.
The Rochester Mechanical fuel injection was offered starting in 1957 Chevys and Pontiacs. I am unsure if it was offered after 1958 until the late 60s. I do recall several Chevy and Pontiac offerings with dual 4 or tri power setups possibly due to the fuel injection unreliability. Olds, Buick, and Cadillac tended to prefer bigger CFM 4 barrels or dual 4 barrels in the higher hp engines as it was more reliable.