I've been at Arunachalam, with my friends, just before the joining date of yours in this forum. We made two Pradakshinas to lord Arunachaleswara on that day. Yes. Arunachala is Shiva himself. In-fact, Shiva purana starts with the story of Arunachala. The eternal Linga manifested at the time of origin of this world became a mountain as Arunachalam. It is Ujjaini, where the goddess Mahakali closed the three eyes of lord Shiva. And it is Arunachalam where she saw him again with his glow.
Another interesting point is, we can see the face of Shiva in the Northern side, formed in the hill. Ramana maharshi also told that, while climbing the hill between Paccayamman koil and Panchamukham, he saw a big leaf coming down from the sacred fig tree, under which lord Arunachaleswara is believed to reside even now, in the form of a Siddhar. Arunachala mahatmyam of Skanda purana also confirms this.
Asyottarasmiñchikhare dṛśyante varabhūruhaḥ /
siddhaveṣaḥ sadaivā''ste yasya mūle maheśvaraḥ //
yasyacchāyātimahatī sarvadā maṇḍalākṛtiḥ /
lakṣyate vismayopetaiḥ sarvadā devamānavaiḥ //
[It's a cellphone taken photograph, But, you can still observe the face of Arunachaleswara, formed below the peak of hill. The location told by Ramana maharshi comes to the right of this face.]
In using your terminology, I believe,
the concept of Ardhanrishvara at Arunachalam is nothing but the state of Advaita.
And Ardhanrishvari of Kamakhya is the union while being dual, a concept of Dvaita.
Lord Mallikarjuna, on Srisailam mountain, is also believed as Ardhanrishvara. His name itself carries this message. Mallika + Arjuna = Uma + Maheshvara = Ardhanrishvara.