Regarding Aduba:
I think we'll have to wait and see. 2nd Stage has a general practice of putting all of their actors below the title, regardless of the role, or the actors' star power. And as a general rule, there's really no point in assuming an actor's category placement based on their billing. The billing is based on marketability, not necessarily the size of the role, and we see the Tony committee rule against the default billing tons of times every year. So, as for the size of the role: they could very well petition her for lead. But I agree her role is a grey area.
Sometimes, in situations like this, a role gets ruled as a "lead" if they're a central figure within the story of the play, even if they don't have more stage time - especially if there's star power involved. So she could be ruled as a lead on the grounds that (a) she's the title character, (b) Aduba has the star power, (c) she begins and ends the play, (d) even when she's offstage, her character looms over the story.
As for Ruth Negga:
Lady Macbeth is a tricky role when it comes to awards. The role is too juicy and too important to be deemed Supporting, yet she doesn't get as much stage-time as Macbeth, or many other Shakespeare leads. And for that matter, the role isn't as large as the other leads in contention for the Tony this year.
Note that the role of Lady Macbeth has never won a Tony award, and it hasn't always earned a nomination either. And even Frances McDormand (an awards darling) isn't in getting any awards buzz for her performance as Lady Macbeth in Joel Coen's film. It's not that these actors aren't giving great performances, it's just hard to win a leading award for a role that borders on supporting.
I like Ruth Negga, and I'm certainly not rooting AGAINST her. I hope she's fantastic. And if she's fantastic, I hope she's taken seriously as an awards contender. But I have my doubts, partly for the reasons I mentioned above, but also because I thought her Hamlet was good-not-great.