2. Since the boot-time scan is essentially done before windows has started fully, it can't update things like the scan history.
It does however update a text file:
C:\ProgramData\Avast Software\Avast\report\aswBoot.txt (Vista, Win7 or later OS location), check this file using notepad for info on the scan.
Depending on your OS - This may be a hidden location so you may have to change the Windows Explorer, Tools, Folder Options, View, hidden files and folders.