If a decedent’s last name is misspelled on the will, is it still legal?

1) Minor typos, like mispellings, do NOT invalidate a will or any other legal document, like a contract.
2) Being made a few days before death does not invalidate a will: someone can draft a will on his/her deathbed, right before expiring.
3) Saturday signing is irrelevant: there is no requirement that wills be signed on a week or business day.
4) Witnesses may be friends of the executor and/or beneficiary.
5) Being notarized in a different county is irrelevant, though the notarization must have been done when signed--you can't notarize after the fact (after signing), and if you can show that it was improperly notarized, that could provide grounds to challenge.
6) Being drafted or signed while ill is irrelevant: many people only think to do their wills while ill.
7) IF you can show by medical evidence (e.g. with the testimony of a physician, psychologist, or psychiatrist who had examined or had been treating the decedent)  that the decedent was not mentally competent when he created and/or executed the will, that could certainly provide grounds to invalidate it.
8) Or if you can show by evidence that it was not in fact signed by the decedent but that his signature was forged, that would provide grounds to invalidate. But you need more than supposition; you need evidence.

that a particular person is, or is not, a beneficiary.
In a Will it is important that the names of both the testator (the person who made the Will) and the executor are such that these parties are easily identifiable, so that it can be said with certainty who they are. Accordingly, for minor misspellings, a Will would not be invalidated.