I agree with all your (excellent) points.

For me, Heath's perf in Brokeback was less about grunting.(Billy Bob? Really? Well, that makes sense...)

It was more about the way country guys in the Australian outback (and perhaps in certain, similarly remote rural locations in the States) talk in a way that keeps their cards close to their chest.

In fact, the flies are so multitudinous and pesky in the Outback that you have to keep your lips close together when you speak if you don't want the buggers flying into your mouth.

So (US accent aside) all that mumbling and muttering Heath did in Brokeback was, for me, so utterly authentic and such Australian bushman-like behaviour, it just made me love his character even more; this repressed, tight-lipped loner.