This could also account for why it seems that anyone can kill Dracula. Let's look at it. The Belmonts aren't the only ones who can beat him.
Actually, not
anyone can defeat Dracula, although there are only a handful of exceptions besides the Belmonts. Hector was capable because his Devil Forgemaster abilities (said to be only second to Death himself, and the original plan was for Death to use Hector as Dracula's new vessel instead of Isaac, who was obviously the "weaker" D.F.)
Then we have Shanoa, who was indeed an "ordinary" human, but was imbued with the power of Dominus, Dracula's own magic which was the only technique she could use to destroy him. Otherwise Ecclesia would've been just another failed attempt like all the other organizations the game's prologue mentions.
Maria Renard's case should be a no-brainer, no explanation is needed.
Nathan Graves is debatable, since he's never actually confirmed as a Belmont relative although Hugh Baldwin hints that his "Hunter's Whip" has some magical ability similar to the Vampire Killer (if not the Vampire Killer itself, perhaps just the game's developers being creative/not doing enough research/mistranslation, but that's solely speculation & we'll never know the answer), so that may have been the sole reason he was able to defeat Dracula.
Henry Oldrey, I can't really speak for since I never beaten the game as him but I chalk it up to purely gameplay purposes with no story-specific reason why he can kill Dracula (not even sure if he even fights Dracula, haven't even unlocked him yet lol)
Albus, as said above can be chalked up to gameplay purposes, but to add a plot-theory, perhaps it's because he had the power of the Belmont relatives he "kidnapped," which aided him in his battle against Dracula.
Eric Lecarde is supposedly part of an offshoot of the Belmont family, so he has the inherent magical abilities that comes with their bloodline. Also Alucard created his spear to "compliment the power of the Vampire Killer." He wouldn't do this for any other ordinary human, imo.
Then there's Old Axe Armor, which can be argued that it isn't even human, just a machination of Dracula's power, so it has the natural ability to combat him lol.
So how can Hammer, who seemingly has no powers, just various weaponry, survive Dracula's castle? You forgot the Merchant from Harmony of Dissonance, which says he just "suddenly appeared in the castle & set up shop" or something like that, obviously via Dracula's magic. Maybe something similar happened to Hammer, and he got lucky that he didn't encounter any of the more dangerous creatures, like the Merchant did. Hah!