Jolie,
I have soaked my amsks up to 24 hrs prior to shaping them. I have also sprayed them and have also used sponges to wet the leather. all work...soaking for 24 hrs(with warm water that cools to room temp...dont use hot boikling water though it will scold the leather i presume) allow you to stretch amnipulate leather easiest though.
the only time i experienced a water stain was when a plant i waterd leaked water onto a belt....
I use treated well water. It has alot of calcium in it but it does not stain leather. thats my experience.
to stretch the leather you could use an egg i presume....but it would be nice to attach something to the egg so that nose protrudes....I would use a hard boiled egg though hahaha or you may stain your leather and stink it up if you use an egg and it breaks hahaha...
even a hard boiled egg would crack i think....a wooden replica egg would work.
I have used soapstone and carved it to the shape i needed for piece before...soapstone is easy to file and carve and is super smooth when polished....but i am set up for that carving so its easy access. Also a thrift store can be your best friend to find odd things to use.
I would also encourage you to use 2-3 oz leather....real thin veg tan...thats what i use on masks....maybe other would vary in opinion though I am not sure.
as for heating the leather....I have not done this...but i have used a hair dryer (blow dryer) for rapid dry ....becareful though you could burn leather i heard...although i have never experienced that.
hope this helps