MacBook Air messing up

After removing the PLISTs as followed above but with the above dialogue box still showing, I called Apple Support. They were really helpful. The support guy said to click 'Show in Finder' and it disappeared. He then redirected me to download https://www.malwarebytes.com/mac-download/. It picked up the StandardBoostD malware and quarantined it. After restarting, all now appears to be normal.

This is Yahoo redirection malware affecting your web browsers. We found this link, https://howtoremove.guide/standardboostd-mac/ and have been able to remove the PLISTs and corrected the web browser from Yahoo back to Google but we have a rather persistent dialogue box which will not disappear. I fear, as we had done, clicking the buttons may attract further problems. We certainly can't restart without holding down the power button. Upon restart, the dialogue box reappears.

'How To Remove Guide' also offers SpyHunter software (free) but alas unable to install as I feel the "StandardBoostd" dialogue box is preventing installation.

While we have made some sort of progress, we haven't eliminated it.