Those are excellent questions. I suppose that it is possible to use ordinary table sugar (sucrose) to increase the sugar content of the dough and achieve certain performance characteristics but I don't know how you would assess the amount of sugar, or the timing of its use, to produce the same results as using diastatic malt. It is important to keep in mind that diastatic malt is not intended as a sugar substitute even though its use results in natural sugar production. It is added by the miller or baker in order to increase the enzymes in the flour that convert damaged starch to simple sugars. Those simple sugars can then be fermented by the yeast to produce carbon dioxide and alcohol and to be available at the time of baking to contribute to crust coloration. As such, the diastatic malt should perform at the same time and at the same rate as the preexisting enzymes (alpha-amylase) in the flour. If the recipe used by the baker calls for table sugar, or the baker deems the use of table sugar to be desirable, for example, for added sweetness in the finished product, that decision is left entirely to the baker.
As you might suspect, diastatic malts that are added to the flour by the miller are in dry form. By itself, and unlike nondiastatic malts, and especially their liquid form, a dry diastatic malt does not add much in the way of flavor or color or texture of the finished crust. For that, you would have to use a liquid form of the diastatic malt. However, the diastatic malt does improve dough handling by helping modify, or relax, the gluten in the flour. It is also fairly rich in vitamins and other nutritional components. By contrast, sucrose is a complex sugar with little or no nutritional value (as you will note if you look at the labeling information on a bag of sugar). Moreover, before it can be used as food by the yeast, and to be used most effectively in the Maillard reactions for crust coloration purposes, the sugar has to be converted to simple, or reducing, sugars. How long that will take depends to a great extent on the dough formulation and the fermentation method used (room temperature or cold fermentation). Tom Lehmann discusses the timing issue of the use of sugar, and when it is most efficacious for crust coloration purposes, in his PMQ Think Tank post at http://thinktank.pmq.com/viewtopic.php?p=26890#26890
. As that post notes, there are enough natural sugars in a dough to support a fairly long fermentation period. That is primarily a function of the alpha-enzyme activity in the dough. Beyond that, the baker can add more sugar, whether in the form of table sugar, honey, maple sugar, etc., if desired or deemed necessary.