2. "Conversation View" is grouping messages together into conversations across folders and even across accounts (in Smart Folder)!
@mikeone: thank you for the clarificaiton. I didn't know that.
(And it's a slight surprise to me, since all other "cross-pollination", including moving messages is not enabled.)
@Kostya:
Thank you for taking the time to explain the technical details. I understand if it is the db schema restrictions. (Although I am surprised to hear that that's the case with your db schema, since it allows you a per-folder inclusion into the Smart folder.)
You might want to make a note to yourself: if you'd ever have a change in the db schema with one of the future releases, you can include the provision for the behavior discussed in this thread.
(I know it is a PITA to switch a db schema, but even Adobe changes it with a major version release of their products, e.g. Photoshop LightRoom. And in that case, the newly installed version offers to convert the old db into the new one during the first start.)
But again, I don't have any personal steak in this functionality.