The specific clauses you listed for your plan have nothing to do with permitting after-tax contributions to the traditional account in the 401(k). It's likely that your plan does not, leaving you with no way to do a Mega Backdoor Roth.
457(b) plans can only be established by government organizations (qualified 457(b) plans) and tax-exempt entities such as charities (nonqualified 457(b) plans).
