1. Impossible with postfix only. There is possibility to achieve this with for example procmail. Set always_bcc to some address, and configure procmail on this account. Procmail configuration won't be trivial in my opinion - you need to recognize somehow which email was received and which one was sent by your server. You may try to use something like below, but I cannot guarantee that it will work in every possible situation.
:0:
* ^From.*(your_domain1|your_domain2|your_domain3|...)
path/to/SENT
:0:
* ^To.*(your_domain1|your_domain2|your_domain3|...)
path/to/INBOX
:0:
path/to/some/other/box
The last one is for catching emails which didn't match first two rules - just for safety. There is possibility, that you will have tune first two rules a little to catch all proper emails.
2. You don't need mysql here, as sender_bcc_maps and recipient_bcc_maps are not the ones you are looking for. They are for specifying which emails (from which sender and/or recipient) are supposed to be copied to your 'backup' mailbox. As you want to copy all emails, you don't need to provide specific addresses.
3. The same as previous - you don't need sender_bcc_maps. But generally - this is just file. You may name it whatever you want. In above example - you don't need to add .cf extension.
4. always_bcc is better. My explanation is in answer for your 2nd question.
5. You don't have to. It's just information, that if you have some canonical/masquerade mappings, they will be applied before copying emails due to always_bcc option.
