Secondly, I can tell from experience that fact checking certain details about the story doesn't take more than a few minutes of one's time. If you still disagree, could you perhaps give some sort of example?
I am agree with you, that to understand even Castlevanian timeline you doesn't need a lot of time. The question is: will new developers even think about spending few hours just to browse through the Castlevania Wikia or some fan sites to checkup on everything? They are not paid for that.
I think that each game can stand on its own in terms of story. I don't really know a lot of things that I wished the story would have elaborated on more like you said, aside from me thinking stuff like "This boss/enemy is very interesting. I wonder what his story is?" or other minor stuff. I like to speculate about things like that when playing the game. I'm not bothered that this is never answered because it's not really important at all to the larger narrative. When I look at some of the things you considered to be holes in the timeline, I thought a lot of the points were rather minor too. Things that would be interesting to know but are not really imporant. If you don't like small things being kept a mystery, you won't like a lot of fiction. Oh well. Opinions, opinions.
I like to have everything explained and neatly arranged in the story. Or at least, to have riddles and unexplained stuff to be integrated in the story from the begining, so unanswered things didn't looked like copout or lazyness of the writer. I am very demanding to such things and I don't like, when writer setup something that could be seemingly important to the story as the whole and then forget it in favor of something else. Reeks of bad writing.
I firmly believe, that details is what make story live and breathe, so to speak. You didn't need to explain some mundainity to the audience, but if you introduce some seemingly random elements, please, explain them at some point and feel the blanks in between. There is nothing more pleasurable, than to see story gradually reveal every part of it and enjoy an understanding of the whole grandiose plot, that was hidden before your very eyes for the whole time.
Of course, I think, I maybe expect from Castlevania too much, since I often look at it, if it was a "storyline game", that it most likely never was in the true sense. Still, I am saddened by this, because Castlevania is kind of unique and interesting tale and with the right approach it could become a very awesome and epic thing. Sadly developers missed one opportunity after another and that's why I am a bit bitter about Castlevania Old Canon as the whole. As of now, in my eyes, it's a storyline of missed opportunities.
As for plot elements that are vital to the story, for example, the mystery of why the Belmonts disappeared and why the gave the whip to the Morris family etc. , I think it was gradually being revealed as the series went along. I was brought up and introduced for the first time in PoR, OoE explained exactly what happend to the Belmont family, and I highly suspect the canceled Alucard game was going to reveal the rest. At least with major plotlines they put an effort in gradually elaborating on them, albeit slowly. So the storyline might not be as random as you may think.
Yet, still it took a lot time to get even to the that point. And there were a lot of things in between, that really weren't needed. Like DOS and Judgement for example.