Coming from someone who's a fan of Maisie Williams, I'm going to be frank with you. I don't see her as Mary Jane Watson. I'd love to see her in the MCU, but I'm extremely hesitant in imagining her as MJ.
As for Hailee, while she wouldn't be my first choice, I could see her being cast alongside Asa as Peter. The two of them have a strong chemistry together, which could potentially work... albeit as off-kilter re-imaginations of both characters. Since Tom got the role, I really don't see her working in the role. Hailee's a great actress, as proven by her performance in True Grit, but I'm not seeing her as MJ.
My personal favorites right now are Annasophia Robb (pipe-dream casting), Liana Liberato, Madison Davenport and maybe Odeya Rush. I'm sure that there's some other options out there that could great too, but I'm not thinking of 'em yet. Of the lot that I've named so far, I think Liana or Madison are probably both the most realistic and best-fitting options for the role.