Can I use smooth santex to cover up rough santex

Of course. Just apply as you would normal emulsion. Bare in mind rough surfaces require more coverage than a standard smooth wall.

yes

I personally wouldn't use sandtex. Dulux weather shield is a much better paint. To cover the ruff paint it might take a few coats

You won't be able to make the rough santex smooth out by just applying smooth santex although it will cover over the colour if you apply enough coats