because he liked it!!
yes you can
yes Matt/Flat paint makes a very good undercoat
It's very easy, acrylic dries very fast, even if you paint thick, so when it's drie then you can just paint over it.
You will have no problem painting satin over top of flat paint. Just remember to use the appropriate paint that will stick to the old coating. If the old coating is latex (water based) you can go...