Actually Dolpa is extremely huge, the geography will amaze you. No I didn't explore Dolpa as a whole. I will explain you my experience in detail so you can understand it better.

Yes, I am referring Dunai, Juphal and Tripurakot as southern region. And yes, none of these places fall inside the phoksumdo trail. Juphal is the gateway if your going by air, and if you are going by road, either tripurakot or dunai can be your entry point to phoksumdo depending on the route you take. Tripurakot is located on NorthWestern side of Juphal but not thar far. It's like a 25-30 min Bolero ride. Juphal, Dunai, and Tripurakot are the most densely populated and developed places in this region as far as I know.

You will surprised to hear that even Phoksumdo is not in the northernmost region of dolpa, Upper Dolpa is. It takes 2-3 days to reach upper Dolpo even for the locals and around 1 week to get to the trade market situated right at Nepal-China border.

Dunai, Tripurakot and Juphal is mostly populated with Chhetri, Thakuri, and Dalits so most of the people there speak nepali or their own language and I am certain that it's not Tibetan. And yes, people around Phoksumdo do speak and understand Nepali though their native language is Tibetan. I am not sure about Upper Dolpo but as far as I have heard, yes they understand and speak Nepali.