You can always install other os along with other browser on a chromebook. You can install whatever engine you want on android.

With ios you cannot do anything, you're just stuck with safari.

Installing other OS is by definition not ChromeOS anymore.

Replacing chrome in chromeOS would make it not chromeOS too. But you can use chromium instead of chrome if you want, and nothing stops you from developing same OS with different browser and installing it on Chromebook. With iOS it is safari and no way to go around it.