In many ways he is like a wizard. How effective he is depends on how prepared he is, what knowledge he has. He could conceivably kill a Lord of the Outer Night with the right information and weapons.
Oh, with the right degree of setup and the right situation, it's not impossible.
But in the Changes situation, they were going to Chichen Itza - on the enemy's ground - on pretty short notice.
I mean, maybe if he shot them in the belly enough to make the blood leak out, they would then be weak enough to kill. But that would require facing them, and I don't think Kincaid could resist their "force of will" ability - as soon as one of the Lords of Outer Night saw him, he'd be helpless.
I don't think a sniper rifle bullet in the head would be fatal to a being like that.
I didn't get the impression they were immortal.
Well... there are degrees of "immortal" in the DV. I agree they weren't capital-I Immortal, as no mantles seem to have passed on. Though they are "mostly retired gods", whatever that means.
But they're "immortal" in the sense of "don't age and really hard to kill". The Red King's hand starts crawling back to him, which suggests a regenerative ability on the level of the "uber-ghouls" in WN. Even if the other LoON are somewhat weaker, I don't think even a .50 caliber bullet to the head would do permanent damage.
Anyway, Kincaid as Dracula's younger brother would be interesting. But I would think he would have more vamp tendencies.
I don't think so. Drakul wasn't vampiric; Dracula joined/created the Black Court to rebel against Drakul. So another son of Drakul would have no particular reason to be vampiric.