Concrete slab pour on a slope

Draw a sketch, those are always helpful.
1. not sure what you mean by plain slab? Are you saying unreinforced, probably not...
2. 1ft cantilever is nothing as long as you are pouring a structural slab ~6" thick.
3. Beauty of engineering is anything is possible, dams are almost vertical and yet they get poured somehow. The question is always cost and depending on where you are, skill. Any slope is possible so the sky is the limit. Lookup a formwork textbook (Formwork for Concrete-M. Hurd-ACI (7th Ed)) it has images of all sorts of concrete pours.
4. As for mix design, you won't ever design a mix it will be the concrete plant that designs the mix call a concrete plant and ask what their stiffest mix is for slopped slabs.