I think (or hope) that that theory has been pretty thoroughly debunked by now. A good target alkalinity for most beers is 0 hence the success of so many in brewing with RO water. This includes some dark beers. For example, the typical Irish stout actually requires some acid to hit what we normally think of as a desirable mash pH. Those determined to keep the old dogma alive will argue that stouts actually taste better at higher than normal pH and if you enjoy your stout more if it is mashed at higher pH then by all means do whatever it takes to get that higher pH.
As I mentioned in No. 2 it is hard to get excited about a few 10's of mg/L magnesium when the malt magnesium is a percent or so of the malt's total weight. But as I also mentioned in that post much of this magnesium may be bound and never make it into the mash. I don't have access to my library right now so I can't look up the magnesium content of finished beer. Then there is the matter of personal taste. This is why I suggested OP try various levels of Mg to see what is tolerable to him.
I would agree on Mg, when in 1.040 or so beer the base malt provides 80ppm or so, if I remember correctly, it's hard to worry about lack of Mg.
But I am still very confused about your argument for alkalinity goal of 0 for most (all?) beers. You (obviously!) know a great deal about water chemistry, so what am I missing?
For example, does it mean the whole talk about adjusting profiles for style, buffering and targeting residual alkalinity depending on style (mostly color) of the beer is not right? This is the approach of Martin's Bru'n Water, Beersmith and Water book by Palmer and Kaminsky - for darker style beers we need higher residual alkalinity of at least >50-100, often higher than 100 - sometimes 150 or so. And if we include decent concentration of Ca, say 50-100 range, this pushes total alkalinity into 100-150+ range for sure.
Are you saying that one can use similarly low alkaline water profile for say pilsner/helles style and a stout - and still get pH right?
A quote from Palmer book, where he looks at brewing a stout with 100ppm (2mEq/L) alkalinity water (66 RA), which he argues is too low for this style
"To brew a rich foreign extra stout, we need a fair amount of alkalinity in the water to balance the acidity of the caramel and roast malts. Otherwise, the mash pH might be too low (~4.9) and the beer will take on a one-dimensional roast character, acidic and coffee-like. This beer should be smooth and rich, sweet and beguiling...
To brew this beer right, we need to increase the total alkalinity and the residual alkalinity. In addition, it would be nice to raise both the calcium and magnesium levels to 50 and 30 respectively.."
John Palmer; Colin Kaminski (2013-04-15T07:00:00+00:00). Water: A Comprehensive Guide for Brewers (Kindle Locations 4224-4226). Brewers Publications. Kindle Edition.
After treating the water in his example, he ends up with water of 150 RA and 183 total alkalinity, which is about 3.6mEq/L - and he is talking how Ca is still too low (40 or so) and needs to be boosted.