If anything, IGA tried his hardest to expand the mythos and enable more games to be released. Call it shoe-horning if you will, but what else can you do? And since the 100 years rule would've made many of the games impossible, I think it was wise to find different ways of resurrecting Dracula, even if it meant that not all of the heroes could be Belmonts.
I mean, maybe they could've been, but in a series as expansive as Castlevania we needed more than just "This-protagonist-is-the-son-of-the-protagonist-from-the-last-game" type of stories. And IGA really tried. With varying results, admittedly. But still.
Now, about the whip. I like using a whip because it feels damn good, from a gameplay perspective
. But a sword wielder here and there is alright with me. I don't fully understand the story behind the Vampire Killer, and why it's only necessary sometimes and other times it isn't. Then again I hardly pay any attention to stuff like that. I prefer to ignore inconsistencies like that or I would probably not enjoy the games as much..
EDIT: Oh, and if you think about it, the vampire killer was never really necessary in any of the games. You can kill Dracula with holy water or knives or whatever you see fit. This goes for the original Castlevania as well.