What animes did they paint and that?

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.

If you really want to ... yes. It might cause some issues later but should work for the most part as long as you apply a thin layer of paint.

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...