(Even if, in some cases, it as just a custom-built SBC running BusyBox, customers still aren't going to go digging through a custom network stack).
P(robably not)
So, the first term in 1) and 2) are NOT the same, and it is quite conceivable that the probability of 2) is indeed higher than the one in 1) (which your pseudo-statistical argument aimed to refute, unsuccessfully).
Imagine any of your friends, family, or colleagues. (Including some non-programmers/hackers/embedded-engineers) What would their answers be?
#2 is WAY more likely than #1. And that's on Android which still has some protections even with a sideloaded APK (deeply nested, but still detectable if you look at the right settings panels).
As for #3; the point is that it's a virus. You start with a webkit bug, you get into kernel from there (sometimes happens); but this time, instead of a software update fixing it, your device is owned forever. Literally cannot be trusted again without a full DFU wipe.
> You don’t need firmware access to install malware on Android, so how many of stalkerware victims actually would have been saved by a locked bootloader?
With a locked bootloader, the underlying OS is intact, meaning that the privileges of the spyware (if you look in the right settings panel) can easily be detected, revoked, and removed. If the OS could be tampered with, you bet your wallet the spyware would immediately patch the settings system, and the OS as a whole, to hide all traces.
If someone brought me a device they suspected was compromised and it had an unlocked bootloader and they didn't know what an unlocked bootloader, custom ROM, or root was, I'd assume a high probability the OS is malicious.