Painting architraves etc

Leif

I had to strip a couple of architraves due to peeling paint, but in other cases I've sanded down to the pale yellow undercoat. Will there be any obvious difference in appearance once both are painted?

Secondly, for the bare wood I intend to apply a couple of coats of Dulux white primer and undercoat (one paint, does both) followed by a top coat of white eggshell. For the sanded surface I assume touch up and bare wood areas with primer + undercoat, sand, then apply 1 coat of primer + undercoat followed by one coat of eggshell.

Does this sound okay?

Lastly, I have some new skirting made from primed MDF. I've sanded away the primer layer around filled screw holes. Do I prime the whole skirting board, or just areas where the white primer has been removed?

Thanks.