How was Abraham Whistler still alive in Blade II?

At the start of blade II, blade runs down a gang of vampires and asks one of them where Whistler is. The vampire states

He shot himself. Then he turned. We just kept him alive!

So here we have confirmation that Whistler does in fact shoot himself, but he is 2 far along in the process to become a vampire and survives, and is that captured by other vampires.

Y’all are forgetting, we didn’t just get the “gunshot” as Blade walked away from Whistler. We see Whistler’s arm fall limb, the gun falling from his hand, as dramatic music played. The part of his body we see next to his arm is in the same position we see during his final converse with Blade. Meaning, the only way someone could’ve interceded was to have something that sounds like a gun, and shoot Whistler non lethally. Possible, but a bit of a stretch given Blade being in the state he was; on edge, adrenaline and endorphins pumping through his viens, heightening his already heightened senses as his rage bubbled to a boil. Blade would’ve smelled and heard the intruders moving Whistler out, unless they decided the gunshot was an acceptable risk, given Blade expecting to hear it, then simply remained hidden until he left. This presents the problems of Whistler continuing to turn verus continuing to bleed out, which would be dependent upon how lomg Blade takes to get ready. A long time considering he melts Silver for his bullets during his montage. Most importantly, how far of a stretch is it to believe a second group/loner just happens to show up at the exact moment to make such a deception possible, and/or that Blade doesn’t smell the presence of other/another vampire/familiar, given he could smell exactly which of the dozen people a block away were vampires by their smell.

This is the best I can do without watching the film again, its been a while...

It is revealed in the second Blade film that Whistler survived his suicide attempt to be kidnapped by another gang of vampires and taken to the Czech Republic. After killing all but one of the vampires responsible for his imprisonment, Blade frees him from suspended animation, taking him to Prague and injecting him with the "cure". During his time with them, he was repeatedly tortured to the point of death, healed, and then tortured again.

Source

So, it would seem, that maybe instead of killing himself, the vampire gang interceded and the gunshot you hear is self-defense. So, no, it is not completely clear that suicide took place and Whistler didn't just reappear back from the dead for a little chemistry with Blade.

since it was made completely clear that he killed himself

Well, there was the sound of a gun firing off-screen, so how is that "completely clear?" Do we know that his arm wasn't grabbed by a vampire as he tried to shoot? Do we know that the gun wasn't taken from him and intentionally discharged by a vampire? Do we know that he did shoot himself, but was still not quite dead when bitten? Did a vampire with a gun shoot the gun out of Whistler's hand so he couldn't kill himself?

We don't know any of that, because it was never shown, and his death was never confirmed, so I'd challenge the basic premise of this.

If you don't see them actually die, then they're probably not dead, unless the actor demands a hefty raise to be in the next sequel. If you see them dead, there's a good chance that they're still not (almost definitely not if it's the monster/killer in a horror movie). If the head is removed or obliterated, and we actually see it happen in all it's gory glory, that's about the only time we can be reasonably certain that someone is actually dead.

The off-screen death is often done intentionally, for the "surprise" plot-twist to come later (use of quotes because it's used so often, it's rarely a surprise any more), as a matter of fact.

TV Tropes: Not Quite Dead (With additional links to other "not dead" tropes)