I’m already well aware of the purpose of the different rests in a multi-step infusion or decoction mash schedule. I’m not asking why you mash at those temps. I’m asking why you only mash in the saccharification range for such a short time. 20 minutes is not a long conversion rest by most anybody’s standards, and there’s no reason to ever hold a protein rest for longer than the conversion rest, as far as I’m aware. If your malt is so undermodified that it needs that long a protein rest (and it’s pretty rare that this is the case with most of the malt on the market these days), then a much longer conversion rest would definitely be in order, to maximize efficiency. The protein rest is only supposed to prime the malt for further conversion; conducting it for too long a time will actually work against head retention, not in favor of it, although I wouldn’t really say that 30 minutes is excessive.
I also have to question what you’re saying about the purpose of the protein rest. Your post makes it sound like 90% if the conversion is done in the protein rest, which is absolutely not the case, if that’s what you’re saying. Also, I have to point that simple sugars have nothing at all to do with body and head retention. All simple sugars do is increase the fermentability of the wort. Complex sugars found in caramel/crystal malts, and dextrins obtained by mashing in the alpha amylase range, add body and head retention, not simple sugars.
Like I said, though, if this schedule seems to work for you, then I’m in no position to question it, but I’m pretty darn sure that your beer will turn out with a more well-rounded flavor if you go for a longer conversion rest, and mashing in the alpha amylase range for a while certainly wouldn’t hurt, either.