It was interesting (and painful) to read back through an older thread that I had posted to while in the throes of trying to figure out what I might have missed regarding Sabers' health.
Here's a new take on "the subject" of grain-free that I've come across since then.
Grain-free IS the ideal, BUT, grain-free in Pet Food speak almost universally means the manufacturers just substitute a starch, instead of meat protein, to fill up the can.
Starch is the new grain. Since we became better educated, Grain-free has been co-opted as a marketing tool.
I am still lame on using my computer efficiently or properly, but I found this angle/information a month or so ago, by Googling and following 2nd and 3rd page results, not the favorably skewed results.
I just found this article; it addresses almost all your points, Omega 3 & 6 etc.
I didn't post it to discuss dry food, I posted it because it has very detailed nutrition info, some of which was new to me even now.
The Truth About Dry Cat Food from Blakkatz
That Grain-free diets substitute a starch, (potato, pea, tapioca, etc.) is being hidden from consumers by well researched marketing campaigns. Natural, Grain-free, Wholesome Nutrition, etc.
Risky GM Potato believed to be in Some Pet Products
What’s Really in Pet Food
The results pages are endless, you can find more explicit starch references, these are just a few interesting sites I found right now.