I completely agree with your point. You should always validate the function of that gun, with that particular magazine, and your chosen ammunition, in the hands of that shooter before putting the gun in service.
I’ve recommended putting 100 rounds of your chosen defensive ammo through the gun, and gotten a lot of blowback on the cost. Well, I tend to buy defensive ammo by the case, precisely so that I can validate the function of the gun.
In several threads, I’ve mentioned my wife’s Walther CCP. It is an interesting gun with a gas piston that mitigates recoil.
But that gas piston, recoil mitigation, system means that it does not like +P ammunition. The higher pressure effectively gives it a stronger recoil spring, which can sometimes induce malfunctions.
So, I validated that the gun works flawlessly with Federal HST124 grain regular pressure ammunition.
I’ve also mentioned the Glock 42 in previous threads. I have no problem with 380 ACP, and the gun is certainly compact.
My former neighbor had purchased a Glock 42, or, more accurately, her husband had purchased her a Glock 42. The gun was really unreliable, she didn’t like it much.
We went to the range one day, and I shot the gun. 100 rounds, her ammunition, all flawless.
Turns out that that small frame Glock, in her hands, with her grip, was unreliable because frankly she was “limp wristing” the pistol.
We worked on our grip. We worked on our shooting technique. The gun got better as she got better. But I think she would’ve been better off with a gun that she could shoot well out of the box. She had no problem with my Glock 19, for example.
So, absolutely, validate the function of that gun, with that magazine, with your chosen ammo, in the hands of the intended shooter.
You are taking too many chances otherwise