Does the legal guardian of grandmother have control of assets when the ward dies?

Check online with your state laws. They may be separate. However when I filed for guardianship, because of the persons assets, I was also appointed by the courts. 
It will not trump the beneficiary because the money goes to its owner. The beneficiary gets what is left after death.

If your grandmother dies intestate (without a will) there will be laws in your state that lay out a formula of who inherits.

No it would not trump a beneficiary.

If grandmother has a will that will be how her assets are distributed. If she needs a guardian she no longer has the ability to change any of these things. If she does not, then her estate would go through probate, with a judge making the decisions about who gets what. You say no other close relatives, you will be surprised how many relatives she has if she is a wealthy woman.

I hope that doesn't change your willingness to help grandma.

No guardian is over just as POA is over at death.