You say the paint is taking well - that's just the trouble with bare plaster it soaks up the paint, leading to extravagant usage. But if you're happy with it, and it dries off OK, then fine. A lot depends on the composition/density of the final plaster coat.
A "mist" coat is all that's needed, Boxers. Same emulsion, but thinned down considerably to taste. Decorators all have their personal preferences for the amount of water. Dulux say 10%, but that's still way too thick for my liking. I usually go for 30 or even 40% water.
Ummmm......... it all depends on the finish of the plaster. If it's over-trowelled so that you can see your face in it, then neat paint will just peel off like a skin. That's why a very thin coat is applied first.
Your chap possibly finished it sensibly, just enough trowelling to leave it porous enough to give a good "key".
I have tried a PVA coat in the past but wasn't impressed. I do give new plaster a watered down coat first. Was told it was the done thing, never had an issue with it. Not experimented enough to know how much of an improvement it is, if any.