'Un-archiver' won't open Chrome Cleanup or Clam ExAve - an Apple approved malware finder. What will open such programs?

I have to guess that you are referring to ClamXav which does not download as a compress file, so my first question would have to be where did you download it from? You should have obtained it from the developers web site which is https://www.clamxav.com/download.html. Once downloaded it should be opened by DiskImageMounter and then you will need to follow these instructions for Installation.

I don't know what 'Un-archiver' is, but it's not the standard Apple utility which is called 'Archive Utility'.