That's speculation. Paar says "Willing or no, they will hear it." There will always be dragons that preferred Alduin's leadership over Paar's. Just as you and I prefer the Empire over the Aldmeri Dominion. The Dragon's have free will and choice to follow who they wish, this is proven not only when Odahviing tells you how many of the Dragons began to question Alduin's Lordship after his first defeat, or when he chooses to serve you because the Dragonborn was worthy of it, but as you mentioned, the Dragon who chose to use Necromancy. The Dragons have as much free will as mortals do, they chose to fight for themselves or under the rule of a leader, like mortals.
He also says
"Many of the dovahhe are now scattered across Keizaal. Without Alduin's lordship, they may yet bow to the vahzen... rightness of my Thu'um." I'm not saying every single Dragon will follow Paarthurnax, just that Paarthurnax isn't rejecting his nature. He's merely trying to dominate through the Way of the Voice.
I'm not sure where you're getting that I've said the Dragons don't have free will or choice? Philosophy is the most important thing to them, it is why they do battle in the Thu'um. Battle and debate are the same to them.
Dragons by their very nature want to dominate, I'd imagine it takes a LOT to make a dragon serve another person willingly, ignoring the illusion Shout that Herma-Mora teaches you, and even that shout doesn't last long.
Except you're not another "person". To the Dragons, you're one of them a Dovah. It isn't a new thing for a Dragon to serve/follow another Dragon. It takes mastery over their Thu'um, proving your dominance. They either accept your Lordship, flee or die.
Every sentient being in Tamriel has free will, they make choices. Paar chose to defy his urge to dominate the humans, chose to teach the Dragonborn, chose to betray his own brother and is making the choice to teach other Dragons the Way of the Voice probably so mortals and dragons can at the very least, tolerate each other if not live in peace. If he really cared to dominate his fellow dragons, he would have taken Odahviing with him, by force, which probably would have ended with you both fighting.
Paar chose to defy Alduin, because he in turn had defied Akatosh. Alduin was only bad because he wanted to rule over humans and dragons, not destroy them all. Now Paar is following the same trap, defying how Akatosh made him to be.
He'd be stupid if he tried to force Odahviing and fight you. Your Thu'um is above his, above Alduin. If he really cared to dominate his fellow Dragons, he'd wait until you were dead. You are the Last Dragonborn. He waited thousands of years, I'm sure another fifty wouldn't hurt.
Paarthurnax doesn't fully trust you anyway, since you're another Dovah.
It is interesting though, the whole Blades vs Greybeards. Paarthurnax believes the Blades to be wise. You're given a complex choice, do you kill the Dragon who helped you? Or forgive him for atrocities so damning they are remembered for thousands of years.
Edit: I wonder if Nafaalilargus was revived. Empire could have their "jewel" back.