If a propose a branch for merge on Launchpad, it will show a diff with the branch that I have proposed to merge with. I like the way the way that Launchpad shows the diff: is there a way I can view this diff on Launchpad without explicitly proposing the branch for merge?
No, I don't think so. This diff is created with bzr merge --preview command and requires 2 branches (source and destination). You can do that locally easily, or you can create a merge proposal and then Reject it.