Having Trevor 1/4th vampire will not effect him negatively by any real stretch. He'd inherit certain qualities like, perhaps, unusual strength and much higher resistant to diseases. And that's on top of his family's supernatural abilities. But he wouldn't have the adverse effects of feeling the need to feed for blood, weakness to sunlight and other such vampire-like drawbacks. This is only speculation of course as I've never heard of any story referring to someone who's only 1/4th vampire. And in terms of Legends fitting into the canon, sure it could. Just mess around with the years like Jorge stated.
Why does every assume the crossbreads automatically get (as Blade put it) all of the strengths and none of the weaknesses? Why couldn't a part vampire end up being weaker than a normal human with aversions to holy symbols and powers, a strong dislike of sunlight (they burn easily), and just generally poor health? You could say that their body simply can't process the blood they'd need to drink to actually gain any of their vampire abilities.
This could also be used to explain in-universe why there's so many versions of the Simon Belmont story: his is one of the most popular tales, and everyone who retells it tries to put their own stamp of contribution on it.
Personal headcannon here. All of Simon's adventures happened, but they didn't happen exactly like they were told. The guy fought a lot of vampires. They weren't all Dracula. He did fight Dracula multiple times, but he didn't always get the killing blow in.
Depends mostly on one thing: Do you want Symphony to still be canon along with Legends? Without SotN, and with a LOT of ingenuity and handwaving and sweat, you can force Legends in there. But it's either Legends or SotN. You can't have both and expect the Lisa plot to work. Now if we go the "we just have to retcon X and Y and Z" then anything goes. We could make Konami Krazy Racers canon.
... It's not?
Okay, but that time Simon teamed up with Konami man happened, right? And the time he went skateboarding in Dracula's castle?
I honestly disagree with the notion of just "messing around with the years" will make Legends fit since there is still the clear inconsistency of Sonia being the first to defeat Dracula which directly contradicts and takes away from the stated fact of Trevor being the first to take him on and beat him, not to mention any shoehorn would take AWAY from Trevor's role in the series as the original belmont to take down Dracula and as big trevor fan I would not be on board with a shoehorned game into the timeline that was never meant to be in the timeline in the first place and was apparently meant to start its own timeline separately.
I think you could give them each the role of being the first to defeat Dracula and it would still work.
Sonia was the first to defeat Dracula. Up until that point he was some supernatural juggernaut that steamrolled over everybody. Sonia showed he was beatable... but she didn't KILL him. She just hurt him bad enough to put him down for a while.
Trevor was the first to defeat Dracula as in, he went out after Dracula and came back carrying the vampires' head on a pike. Dracula was gone. He was defeated. For good. (not really)
And as far as the IGA was a huge fan of Castlevania... I'm not sure I can fully agree with that. he was a huge fan of what he made Castlevania into, but what it was before he started and what it was after are two wildly different things. I'll give him the fact that he had a vision for the series, but that vision turned it into something it hasn't before. I won't argue that it was better or worse than it used to be. (I might do that later) but it was hardly the same. It's easy to see how people who started with the tributes to Universal monsters and Hammer Horror could dislike the more flowerly Anne Rice style vampires or the more anime inspired stylings of the series.
Also most of IGA's attempts to give things origins have really sucked.