1. any zipped/archive file, password protected or otherwise has to be unpacked (this would create new files which depending on the file type would be scanned) and any executable file run (again avast intercepts the execution and the file is scanned, if infected avast alerts and doesn’t allow the file to execute). So yes there is every point in an AV.

  2. the only files that should remain are usually .dat files but they are text files which just contain info in the files in that temp internet files folder. There is nothing stopping you checking to see if the temporary internet files are deleted when you choose that option.