How can I abort a High Sierra installation?
You should be able to safely abort the install. Quit the installer without reboot. Then open Terminal.app and remove the installer folder (SIP has to be disabled to remove everything - most files and folders are deleted though, even when SIP is enabled!):

sudo rm -Rf /macOS\ Install\ Data


Then enter

sudo systemsetup -setstartupdisk /System/Library/CoreServices


A quick scan revealed that the only file/bundle installed/replaced (except some receipts) is /System/Library/CoreServices/MRT.app

Its version number is 1.23. The original file's version in macOS Sierra is 1.22. MRT is (Apple's) Malware Removal Tool.



If SIP is enabled I recommend to execute the steps above. Then immediately boot to Recovery Mode and disable SIP. Then reboot opening the Start Up Prefs and choosing your main volume. Remove the remnants of the macOS Install Data folder (which should be possible now).

Reboot again to Recovery Mode and enable SIP again. Reboot to the main volume.



Apparently Sierra's MRT.app isn't available in any Sierra Combo updater. You would have to extract and re-install it using Pacifist and the latest macOS Sierra installer.

High Sierra's MRT.app may also work in Sierra; so it's unclear whether installing the original version is really required.