First USAA is a rather fine organization and I doubt they are playing games with you by refusing to make distribution directly to the beneficiaries named in the Will.
What looks like a Will, says it is a Will, and feels like a Will, is NOT necessarily valid, nor the LAST Will (believe it or not people repeatedly come up with later wills), or that there may not be creditors that come up out of the woodwork or taxaes payable. If there would be any of these claims against the estate. The probate process is what is used to "prove" the document is in fact her last Will, there are no challanges to it, and any claims against the estae are adjudicated under court supervision.
If the account was solely in her name, it now is the property of her estate, and only the executor named in a Will AFTER THE APPOINTMENT IS CONFIRMED by the probate court, or a Personal Representative of her estate designated by the probate court, has rights to the money. Further if the bank paid a beneficairy and any creditor did not get paid, or any Fedreal estate tax was not paid, the Bank could be held liable to the creditors/tax authorities if it did anything else. Sorry.
In some cases it is true an institution may waive the requirement that it be probated IF the beneficiary is the principal heir at law, all other possible heirs at law have signed waivers and authorizations to pay the money to the beneficiary, and have agreed agree to indemnify the bank should any claims be made.
But that's the exception, rather than the rule for nationally operating instiutions.
For the bank to pay the person named in what you say is a Will (and I believe you) mean the Will is is NOT dispositive