I, too, have always believed that the REAL story lay with Marr. As far as I can tell, Joyce and Rourke have always had a deep respect and loyalty for Morrissey, regardless of the lawsuit, but Marr has totally been loyal to Morrissey and his mystique. I would imagine he would still remain loyal in an autobiography, but what Marr could reveal would be essential. They definitely had a relationship, not sexual obviously, but a marriage of sorts, as is the case with most good or any songwriting team. Just the fact that Marr sat in a room with Morrissey probably alone, together, is almost enough for Marr to have picked up inner details that no one else could and may ever know. I suspect Morrissey has always been on his best guarded behavior, never letting anyone really know the real Moz, even in the 80s. But even the most heavily guarded castles have vulnerabilities and reveal the "inner you". Marr of all people would know this, as I feel that Morrissey held Marr (and still does) very near and dear to his heart. Of course, this would only really apply to the 80s and who knows if Marr can really remember the details, it seems he's tried to block it out. I read somewhere that Marr might've said, I'm not sure, that Morrissey was to blame for leaking the "Smiths To Split" rumors after, of course, Marr had already stated his departure.
Other people with insights would be: Alain Whyte (who, as far as I can tell) is VERY protective of Moz AT ALL TIMES. Boz Boorer, while a little more quirky and willing to slip, collects himself, usually, and lets NOTHING slip. Spencer Cobrin definitely felt hurt by Moz, if what he says is true about the infamous "FAX" regarding is race. And there are others, all who remain silent. Morrissey seems to inspire fierce loyalty in most people. The fans, obviously, but the ones close to him too which is a testament to the quality of person he is.
Back to Marr, it would be great, an autobiography, and I believe Marr to have the goods, but the likeliness for it to come out in an autobiography are slim. It still would be a good read, but the nuts wouldn't be there, I assume.