Existing colour paint is not terribly old, and no issue with damp wall right?
Just paint over with matex for A4 paper white.

After ten years my matex still hold up perfectly.

This is usually done if the existing colour is dark or diff colour, to "lighten up" the background so the satin glo colour can "pop" well.

Brand preference only lah, Matex do not dilute with more than 2-3% water *be very kiamsiap with the water for best results*.
KCC more solids but harder to stir.. it dries thicker
Matex very beginner friendly, easy to apply, my personal preference
Jotun.. somehow I find their economy paint not as cheap so I never use

Yes otherwise cement suck up water like a sponge and you will definitely see the line there.
Easywash for colour paint.. else any matex also can for white. If it stick properly to the wall its quite easy to wipe down with soap and water.. I use economy white for kitchen wall and frying stuff the oil splatter everywhere so no point for me to use good paint.

Enough to cover the bare cement mortar (hacked area) plus a lil on the sides. No harm to apply more one.

You're painting on good condition existing paint so no need sealer. It would be good to paint over with economy white so that the true colour of new paint will not be "tinted" with dark old paint lah.

usually i just use the "White" one.. 15425 is ICI code for white IIRC.. both should be A4 paper white aka boring white, either is fine