It's likely only those with the "Belmont" surname ARE counted as the rightful heirs. Remember, especially with notable families, passing the family name down and ensuring the name's survival was considered important. Even regarding us fans, we tend to get giddy MORE when they announce a new BELMONT character rather than a non-Belmont character.
As for Bram Stoker's Novel being definative canon, it's a yes and no. Events within the novel took place in CV's timeline, but events in CV's timeline didn't take place in the novel's timeline. Get it? There's also room for these events to happen differently in CV's timeline than they did in Stoker's novel. The only thing they share, likely, are the characters. For all we know, in CV's version of the events, Quincy ventured off in traditional Belmont way, trekking through the Transylvanian countryside, through Dracula's castle and defeated him just like, say, Simon would've.