A good exterior grade semi-gloss or satin in any colour that appeals to you will work fine.
Many hotels that use paint use satin or semi-gloss latex paint in their interior applications.
If you want to turn your satin paint into flat wall paint simply mix one to one. One gallon of flat to each gallon of satin. The flat wall paint will obliterate any sheen the satin once had.
Yes
If they are both of the same base you can. Satin paint can be an acrylic or a latex,or an oil. Check on the can cleaning instructions for base. Jason Pollock used both in his work 'Bluepoles' which hangs in the Canberra museum I think
There are different kinds of satin paint - you'd have to know if it was latex, oil, enamel, alkyd etc.
Yes, as long as the surface is free of dirt, wax and grease, and the eggshell and satin are both the same type, ie latex or oil, then you should have no trouble at all painting a satin sheen paint over an eggshell. Also, if the base is water based and the topcoat is oil, that will work fine too.
Add a little talcum powder to the paint.
Satin is lower gloss than semi-gloss paint. Paint finishes in order of decreasing gloss are: * Gloss * Semi-gloss * Satin/Low sheen * Flat Some manufacturers call Satin/Low sheen finish Eggshell, and others regard this as a finish between Satin/Low sheen and Flat paint. The higher the gloss finish the easier it is to clean, the higher its durabillity and the more it shows surface imperfections.
No, you can dull satin or semi-gloss but not the reverse.
Any paint used with a paint base will work well to cover the existing paint.
If you have the satin, you can add flat paint and take some of the sheen off. This would have to be done before application. It will never be a true flat though.