All the scannings run into the avastUI.exe process by design, as far I know.
This will have to change for me to continue using Avast as my AV client.
ClamWin has this functionality (on-demand) without the service, so I could use Avast + ClamWin (and disable the per-user AvastUI process), but I might as well find a program that has both the service as well as an on-demand scanner.
(This is not an attack against you,
Tech, but a comment for the developers.)