The best way to install a hardwood floor is to investigate just what all the layers of underlay and subfloor are made of. It does you no good in the long run to install a wood floor over an unknown subfloor. If you happen to find OSB or particle board in any of the layers, these will have to removed, as the flooring nails ( never use floor staples) will work them selves loose from these materials.
That all said, I know it is more expensive and time consuming to remove the old lino. But you really only need to remove a small patch of it, to confirm that it was laid on a PLYWOOD underlay, and a PLYWOOD subfloor. If this is true just renail all the layers of plywood, subfloor and underlay with some really long nails that penetrate to the joists. This is the only thing that will prevent squeaks. And as long as the vinyl is the flexible kind it is all right to lay the hardwood on top. But like I said make sure all the layers under the vinyl are Plywood.
And as to your last question, get a sand-on site floor, done with a finish that you can recoat yourself. For me it's Oil Modified Poly. No finish in the world will resist the scratches, but the OMP will be easy and cheap to recoat. If you want to try a newer but as yet time proven water based finish, read my article in the Floored News about the Dura Seal 1000 water based finish.
But in all fairness to the prefinished floors, you can read my Melrose article in the Case in Point section, and my new Prefinished article in the Floored News also. But the wood under these remarkable finishes will get dented by the dogs claws too.
As always your Most humble servant, Joseph, the Wood Floor Doctor.