Damn not dragon girl genocide, the world needs more of them, that said between Voldars corruption ability and Alvinias raw power shouldn't they be able to just purge the dumbest among them, and leave the rest under their care, be their nature covetous and violent, nurture has a way a bending and breaking nature, doubly so if one has harsh as the two of them are going to enforce.
Mmh... I think you have missinterpeted this particular part. You say Alvina's raw power... but that's not actually true.
First of all, Alvina is not immortal just as he isn't. Aka... both of their bodies can be destroyed. They share a SIMILAR concept, such as Voldar like his kin is capable of coming again after regaining strength and potency in his spirit, taking form and shape again. But this process can last from hundred to thousand of years like it did this time, cause of how screwed he got from war.
Not to mention he used half of the power he had to affect his own memories, deleting true ones and creating fake memories. But... that'll be revealed in CH2 start, aka as to why he did that.
Alvina? Alvina gets reborn again... as he said, can't kill her, just unmake her. And none can do that except the big guy. She's like his nail, and you can cut the nail down... but it just regrows back. But you've not properly read the part where Alvina said
''I can't fight this. I'm old... as the world changes so do I."
Alvina is in somewhat a similar situation as Voldar is, far from the glory days. Just that unlike Voldar she can be a dragon size of a fucking mountain, spewing fire all about the damn place. So she has advantage in that part. Doesn't mean she doesn't tire or can get hurt and her body harmed.
Get enough dwarves, and they'll chop her down like a tree with their axes and their machines. Raw power doesn't make you invincible. Alvina's saving grace is her age and wisdom with it. Unlike Voldar, she is a master in warfare and tactics.
Voldar on the other hand? If he gets his body destroyed he is done. Game over. He'd be reduced to a shade somewhere under a tree. He spent half his remaining power to break from prison and take shape. That is why he is so subtle, paranoid and cautious in what he does. That's why so much scheming and plotting, and why he just doesn't go and stomp on everyone himself.
So no, they're not really THAT powerful as they seem on surface. They got huge ass risks. Also... I will spoil this particular part, or rather I will MENTION it, since it gets mentioned in CH2 anyways.
If it was a matter of people that inhabit the world alone, and their races. MAYBEEEE they would be less screwed... but later down the line Voldar has to fight two wars on TWO FRONTS, entirely two different wars. So yeah, he doesn't have the benefit of fucking about and taking additional risks and chances that would cause his demise.