Batman:

 

but when you reply to those email from the catch-all mailbox, how do you reply as that specific alias for that specific email?

 

there is no "reply as" option in the email compose box.

 

When you're replying, use "full editor" by clicking on the 'reply' dropdown, select "Edit subject" which will expand the compose email, click once on the email "To" field, and the "From" field will show up and you can change the reply-as.