Is there a way to view all the messages in a folder containing subfolders?

I assume the folder and subfolder you refer to are local mailboxes created for specific clients and subfolders within. If so, how are you moving them to these mailboxes?

In order to view messages in a local folder and also a subfolder, you would have a copy of the message in each folder. If you are using rules to accomplish how they are moved, you could set it to copy instead of moving the message to the subfolder. That would mean more space for storage when duplicating emails.

There is a Smart Mailbox option.

Smart mailboxes are more of a search feature with preset criteria. Messages remain in the mailboxes they were stored in, but can be accessed in the smart mailbox. You can also create Smart Mailbox Folders in which you can put sub smart mailboxes within like you do with regular mailboxes. This seems like it would be a much better option for you.

No, you can't mix "all" and "any" filters. You can only select one. I haven't had a lot of success using boolean for filtering. Only thing I can say is try it. If it works for you in the Mail search window, it should work in the smart mailbox search.

For creating search criteria filters, only thing I can suggest is think about what you want it to show. Then look at the criteria given in smart mailboxes and think about what the best way to achieve it would be using the any or all. You may need to create seperate SMs for different criteria. As I said, you can put smart mailboxes in a smart mailbox folder, but you can put a SM in a local mailbox or within another SM.

You could also use SM in conjunction with the local mailboxes. If you are moving messages to seperate local mailboxes, you could just use the SM to view all messages from a particular client. That would make more since than moving the message to a main local mailbox and then copying them to subfolders so they can be viewed from both. The big downside to that is duplicate messages, but if it works for you then that's OK.

I haven't tried boolean search since first installing Lion. I didn't have good results from it back then. It might be better now.

Here is an article from Apple on using search feature in spotlight. Try them in SM and see if it works there.

http://support.apple.com/kb/PH11176?viewlocale=en_US&locale=en_US

Also, open up the help menu in Mail and type in boolean search. It will give you some info there.

If that doesn't give you enough info, you might also try google search on using boolean search with Apple ML.

I don't think so. Don't worry about it.

Good Luck