In Texas, is it possible for a step-grandparent to adopt his grandchildren where he and the natural grandmother have had managing conservatorship of the children for 2 years, both children are over 2 years of age, one parent, the father, has consented to termination, but the mother is contesting the termination of her parental rights?
If we fail on the termination, but could get her to consent to the adoption, is it possible for the step-grandparent to adopt so that he could name guardians for them in his Will and leave his estate to them without the birth mother getting her hands on it?

As a practical matter, if the mother contests the adoption and does not agree to terminate parental rights and she is not so bad that the court terminates her parental rights, there will be no adoption. You can continue to have the kids in your current status.