random email tips

here tells you how to remove a message in a conversation in gmail

however, it seems that it's not possible to remove attachments from the messages in gmail

it's also not possible to set automatic bcc option in gmail; in thunderbird, in "account settings" -> "copies and folders", specify the address you want all the message to bcc to

this page contains tips on how to remove an attachment from a message in thunderbird

to search for unread mails prior to some date, try searching with multiple conditions:
label:unread label:inbox before:2007/1/1