Howdie do fellow friends,
I was struck with this invasive search InstallMac engine as well,
I just came back to share the method that worked for me!!
Because I tried restarting Safari and clearing history and resetting preferences and quitting but my top right search bar was still not going to Google, in fact even the bing search engine stopped.
Ok I went to HOME folder tab, on the left sidebar in Finder,
Then I went to LIBRARY
from there go to PREFERENCES,
then in that folder i found a few 'plist' files, but mainly I was looking for anything that said InstallMac,
I deleted one file in abbreviated form that looked like InstlMc or something, then two more that said InstallMac something plist.
So i deleted three files, reset Safari, and my search bar works and automatically defaulted back to Google!
So i guess that worked, I hope the same method works for anyone trying to remove this satanic entity,
Also don't download any software removal tools, Obbvviously, I consider them in the same pile as this crap InstallMac software.
Anyways Goodluck!
Hope it helps 😀