This is actually one of your best questions, DWTL. Kinda reminds me of Justice Holmes' rationale for forced sterilization: "Three generations of imbeciles are enough".
Practically, I doubt the USSC would ban abortions for anyone. However, I think the USSC might allow greater state regulation of abortions procedures on mentally unstable or retarded people.
No it's not - it's the opposite.
And I think all abortions should be banned - ESPECIALLY if the person wanting an abortion isn't mentally up to par.