A few notes on dry roux (or 'browned flour' as some here feel compelled to call it
First, oil turns flour a few shades darker, so if you take the flour to a brown paper bag color, when you add fat to it, the color will actually be quite a bit darker than that. You always want to take dry roux to a shade or two lighter than it's wet counterpart. Black chef, that could be why your Mother's dishes were so much better- by toasting the dry roux to the same color as if it had been wet, she's toasting it quite a bit more.
Fat/oil is an excellent conductor of heat. When you make a butter/flour roux, the butter carries a great deal of heat to the particles of flour not in contact with the pan and, in turn, the flour cooks much more evenly. In a dry situation, it's extremely easy to have some browned granules and some completely raw. Although raw flour isn't the end of the world (it's used in beurre manie), it's not as tasty as browned. So, toasting the flour evenly helps the flavor in your end product. The only way that I found that browns the flour perfectly evenly is to lower the heat and stir the flour like a madman. Baking can be problematic when it comes to even toasting, even with frequent stirring.
Should you decide to add dry roux to sauces without combining with fat first (for fat free cooking) the fat in regular roux keeps the starch granules separate when they hit the liquid so they don't clump. Without it, dry roux has a tendency to clump like crazy. Three things help:
room temp or cooler liquid/cool dry roux
whisking like crazy as you slowly sprinkle in the roux
cooking the sauce a little longer to help the clumps break down
As far as storing dry roux in a jar for extended periods of time... I wouldn't. Flour contains trace amounts of fat. Fat + air + heat + time = rancidity. When you toast it making the roux, you shorten it's shelf life considerably. I've tried storing dry roux in a glass jar and when I opened it a few weeks later, I had rancid flour on my hands. Not recommended.