Is it legal for a beneficiary to sign as a witness to my signature on my Will?

It's legal but not looked upon favorably. For good measure have someone outside of beneficiaries sign in front of a notary to avoid confusion in the long run.

No, it is not permissible for a beneficiary to sign as a witness, which is why it is a good idea to choose mere acquaintances who wouldn't expect to inherit as witnesses to your signature (rather than relatives with great expectations). If a person who is a beneficiary under the Will signs, this doesn't invalidate the Will, but it does invalidate the bequest to them, which will fall into the residue.

It's legal for them to witness the will, but they won't inherit under anything. You can be a witness or a beneficiary, but not both. Get someone else to do it.

At common law that would nullify the will because questions may arise to whether the author of the will was coerced or not.

No as their could be a conflict of interest

Have it notorized. (in case there's a dispute for any reason. that is a "conflict-of-interest")