But I isn't Muir's name John, not Jon?
However, "Jon" might be Jon Savage, the music journalist/critic from London who lived in Manchester at the time, who used to publish the fanzine called "Secret Public" with Linder, and who later wrote the well-known book on punk in Britan, "England's Dreaming".
I have no idea if the letter is genuine or fake. As for Morrissey travelling to Texas, I know he did go to USA to visit his aunt Mary who lived in New York a few years earlier, and accroding to "Severed Alliance" it was the first of his several trips to USA. Whether he went to USA in early 1981, and whether he ever went to Texas, I don't know.