Just won't have e-mails who has an infected file in it or in attachment.
Ok, choose 'No' and the infected email attach will be sent to Chest.
In the home version it is only possible to send the infected files/mails to the the chest, the pro version has more options.
Nope... You can choose 'Yes' and delete the file inconditionally.
As I said before, it's better choose 'No' and then, after, delete the file from Chest. A deleted file cannot be recovered in anyway. A file sent to Chest could be recovered. Remember that, sometimes, you can have a false positive (a file that is not infected but avast! detects is like so...).