I said that MYN was a perfectly reasonable substitute for MINE but unfortunately I was disbelieved, even though I pointed out that MY has a Y not an I.

Well, there is no rhyme or reason to English orthography, and you are "free" to spell words any old way you want. Whether that "freedom" is backed by some right, who knows? That being said, folks will judge you by how well you follow the standard orthography, just as they will judge you (rightly or wrongly) by a host of other arbitrary conventions and accidents of nature. I suggest one of you adopt the Middle Welsh dotted y: ẏ.

English during its history has gone through relative periods of orthographic stability and chaos. Shakespeare's time is a good example. Take a look at a page from one of his plays in its various editions (link).

Ceci n'est pas un seing.