Is it permissible for a man to wear a diaper-like covering for the awrah, without stitching, under the ihram garments, knowing that it is now sold with ihram garments, made of cotton fabric, and fastened with snaps and elastic?

The "makhit" (stitched garment) forbidden to the pilgrim in ihram is that which is tailored to fit a limb of the body, even if it does not involve actual stitching. Some people err in their state of ihram by wearing something that resembles a makhit, such as fastening the rida' (upper garment) with buttons, making it akin to a shirt.