How much gluten for soft, airy, enriched dough?
You certainly don't need that much gluten for a brioche. A brioche is in fact somewhere between bread and cake, and very different in crumb from high gluten breads. 14% is not even typical bread flour, it is probably bagel flour. It will give you a gummy, resilient crumb even if you manage to bake large holes into it, while for the brioche you want cottony softness with just a hint of springiness so it is not like chomping down on а low quality sheet cake. 

First, the amount of gluten in the flour. I have successfully made great brioche style bread (kozunak) with AP flour in the 8-10% range. Kozunak is supposed to have well-developed gluten, which in my experience cannot be substituted by using a flour with high gluten content. It is achieved simply by long kneading by hand, using a good stretch-and-fold for optimal alignment. This is less needed in a French brioche, so you can knead by machine. 

Salt does not inhibit gluten, on the contrary it makes it tougher. And sugar is probably difficult to incorporate later and still have it dissolve well. So I would say that you should follow a standard recipe instead of tinkering with the process, so you will probably end up dissolving the sugar in the milk, or foaming it with the eggs, and adding the salt from the beginning. No need for separating the eggs either. You can even use a yolk only recipe for maximum tenderness, but note that the combination of high fat and pure yolk results in a very squishy bread that kinda disappears in the mouth cotton candy style. Start with a good recipe and then experiment with the ratio of egg to yolk from there, if you want to go in that direction. 

For extra tenderness, you can try using lard or other solid fats instead of butter, but you will be changing the aroma too, getting something which is still a very tasty bread, but not a classic brioche. Using a liquid sweetener is also possible, but not entirely necessary. 

In short, the big change you have to make is to indeed use the proper flour, which would be a very finely milled AP flour. That would overshadow the effect of tweaks in process or ingredient substitution. Once you have gotten the baseline right, you can go into these tweaks to get your recipe to perfection.