soaking before you wear them can lose dye to minimize bleeding (hot soak), and the denim does get softer i think after you soak/wash them. they'll shrink if they're unsanforized. if they're sanforized, then shrinking will be minimal if at all. (sanforized denim is denim that's been washed or manipulated or stretched or something by the factory in order to essentially get the stretching out of the way) i'll quote a part of a post on a thread i found from google
Sanforizing denim is a method of stretching and manipulating the cloth in the factory prior to any washing so that any shrinking during future washes will be minimalized. It is important to note if your raw jeans are sanforized or not before determining what size to buy, non-sanforized jeans will shrink 7-10%, while sanforized jeans will shrink 1-5%. It is often advised to give non-sanforized jeans a warm soak before wearing them to get the shrinking done before you create wear marks on the jeans.
and i can't help you with that last part bc i don't know what elastane in denim is lolz