Muniment of Title does not get you letters testamentary, and from what you describe, I can't tell where you are in the process and what needs to be done. If the lawyer is not available after paying $15000, then find another lawyer. You need a lawyer's guidance. This is simply too complex to answer in this type of forum without a face-to-face interview.
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Attorney Pyke is correct. It is very difficult to tell where you are in the process, but as you can tell, it is a maze. You need a solid probate lawyer at your side to guide you through. Good luck to you.
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I suggest you contact a board certified estate and probate attorney in Dallas who can advise you regarding what needs to be done once he understands the actual status of the probate proceeding, which is not clear from your post.