First off I don't use the swipe function. It appears that the swipe will only archive and can not be set to delete.
Long pressing an email item will select it (or tapping the email's icon). Select as many as you wish. Watch the third icon in the upper right. It should show an open envelope. If not then be sure it does or all those deleted emails will be included in the is:unread search. It was not clear what that third icon did.
