I myself like matt finishes as the satin and silk always look to shiny. I ver recently been using Tikurrila Durable matt which is supposed to be 10,000 times scrubable. Ideal for pets and places that can get splashed. Dulux also do their own Clean Extreme, which is a similar type of paint.
The decorator is an idiot. You need a mist coat on raw plaster before anything else gets done. A thinned coat of white matt emulsion is all that is required prior to giving it full coats of your chosen paint. The suction on new plaster would draw the moisture out of un thinned paint and it would...
The Gardz is a specialist product which once applied anything can go over it. Paint or paper. Lots of modern sealers are water based, even for water stains.
Doesn't have to be spot on that ratio, but the first coat has to be thin to penetrate the plaster. Its not meant to cover the surface to be solid white. The coats after the mist coat can be un thinned until your happy with how solid it looks.