Pretty sure rclone uploads just fine without server dependencies, yeah. I never installed anything special on my home NAS and it happily accepts uploads with rclone.
That I can't really speak of. I know it does not reupload the same files at least (uses timestamps) but never really checked about only uploading file diffs.
Nothing offhand, but basically it can't know what's on the server without reading it all, and if it can't do that locally, it'll have to do it remotely. At that point, might as well re-upload the whole thing.
Its front page hints at this, but there must be details somewhere.