It's a good pose, but the leaves are a little worse for wear!

Yes and no on the arse-licking. Unlike with dogs, where arse-sniffing is the standard way of saying hello, in cats it can be a bit too up close and invasive of personal space. So the cat doing it might be somewhat impolite and boundary-pushing, but the fact that the other cat tolerates it says a lot. Definitely trusting!