Not completely, the web shield intercepts traffic before it gets into your browser cache/hard disk, without it the scan newly created/modified files option of the standard should scan files as they arrive on your HDD.

Depending on the settings of the standard shield, Customize, Scanner Advanced, see image, the Red Line shows the area of interest and the Red arrow, if that was checked every downloaded file would be scanned. I suggest a browse of the avast help file.

However, disabling the web shield I would say is removing a symptom of the underlying problem, which we haven’t established.

Can you answer the other questions please.