Yes, you can. You can always paint over with same paint.
You should never paint a deck, the paint will peel (unless you live in a very arid climate I guess. The deck may be stained, you can put either oil or latex stain over old oil stain.
Solvent based.
Yes. You can paint semi-gloss latex paint over flat latex paint. It is not advised to paint over any latex with any oil-based paint. Nor is it advised to try and paint over semi-gloss paint with any other finish without first de-glossing the finish and priming.
the high gloss acrylic would bead up and not give you an even application over the oil based
Yes, but you should scuff the gloss coat with sandpaper first, and you might need two coats to kill all the shine.
Yes as long as you use the same base paint. You just need to sand it enough to where the paint will adhere. Make sure if the original paint is latex, use latex. If its oil based, use oil based.
High gloss Exterior Oil-Based Gloss Enamel over Quality Primer will give the best results on a cornice.
No, often high gloss paint is oil based.
Yes, you can.
can i use water base paint over oil base paint
What they call "oil base paint" is really called "alkyd base paint." So yes, you can do that.
Yes, you can