On to the 9x18...
"However, 9x18 was most certainly intended."
Actually, it wasn't.
The 9x18 project had its origins in the Soviet capture of the Walther works in Eastern Germany.
One of the projects that they stumbled across was an early 1930s Luftwaffe project investigating the greatest performance that could be wrung out of a Walther PP-sized blowback operated pistol. The project was nicknamed the 9mm Ultra.
Intervening war caused the project to be shelved.
The Soviets ran with the project after the war. Their solution to increased performance was to increase the diameter of the bullet by making the case a true cylinder (not the 9mm's tapered cylinder), decreasing bullet weight slightly, and upping velocity a bit.
The 9mm Makarov round was adopted in, IIRC, the early 1950s.
Now, you say that the Soviet intention was backwards compatibility.
You're wrong, and here's why...
What would be backwards compatible with the 9mm Makarov? .380 Auto/9mm Kurz? The .380s case is shorter and the bullet quite a bit smaller in diameter, both of which would tend to give problems with chambering, potential misfires, and accuracy (there wouldn't be any).
Using standard 9mm Luger ammunition in the Mak is also a big no go. Again, chambering issues with a longer case (generally the 9mm Luger won't feed through a Mak's magazine) and the Luger operates at significantly higher pressures, very likely leading to gun damage. Not a good idea at all.
Additionally, exactly with whom would it have been backwards compatible?
No one was issuing .380 autos as standard military armament in the 1950s. The United States had its .45, and every other western power affiliated with the United States/NATO was issuing 9mm Luger.
I'm sorry, but the myth of "we can use yours, but you can't use ours" is exactly that, a myth. It had no bearing on the design or development of Soviet small arms/ammunition at any time.
What it really means is that the Soviets let the west do a lot of the hard development work on new cartridges, and then they came in and put their final thumbprint on it to meet their perceptions of what makes a suitable cartridge.
One last word on the 9mm Ultra project, which the Soviets turned into the 9mm Makarov.
In the 1970s the Ultra project was revived by Walther for the Super PP pistol, and renamed the 9mm Police.
As with the 9mm Ultra, the objective was to give the highest level of performance in a blowback operated gun. This was during the convulsion of terrorism in Germany, and their police forces, still armed with .32s and .380s, were finding themselves in need of more powerful guns.
As it was in the 1930s, the project was a flop, and the guns and ammunition faded out pretty quickly.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.