Q1) I'm pretty sure that this is a function of how Gmail IMAP works, not your IMAP client. The Gmail web interface is "smart" enough to pull the rest of the conversation from All Mail in order to show it in the conversation, but an IMAP client would have no way of doing that, especially since they're kept in different folders. So the answer is no, RoundCube does not do this.
Q2) Yes, RoundCube will mark each message as read when you read it. And if you decide to use the Preview Pane, it will even mark them as read as you Preview each message, which is nice if you want to avoid switching back and forth between views.