Martinb

Walls should be painted first to avoid drops of paint on the skirting boards. also emulsion paint dry quicker so skirting boards can be painted few hours after the walls are done.

Lining paper, wallpaper, oil based paint and loose sections of the old plaster should always be removed before plastering. otherwise it may start coming off once plaster starts to dry

Hi. either way of doing it is correct but applying mist coat first (paint mixed with water and let it dry for a few hours) will make rough/imperfect areas more visible for you to sand them down so i prefer second option. also keep in mind that after sanding those areas will loose all of the colour and become raw plaster again so all you need to do is paint over them again (paint&water) before applying a proper coat of paint.
my number is 07857083147 if you have any further questions you can text or call. regards

Hi.
you can wallpaper over emulsion paint but keep in mind that the wall needs to be sanded down to remove any dirt and imperfections and holes needs to be filled up before applying wallpaper. also after preparing a wall it's good to paint it with wallpaper adhesive or pva and let it dry before you start wallpaper.

Hi.
is it reveals around window you're asking about?

plaster will chip off the wood nearly every time.
because of that plasterboard should be applied on the top of it if possible.