If you don't know the nanny personally, don't hire her. It is in that situation where a child is alone out of the sight of any other adult that abuse happens. Daycare abuses happen occasionally too, but there are witnesses.
Why do you suspect your inlaws aren't providing good care? Usually grandparents are much better caregivers than nannies from an agency.
Most of your other questions would probably be best answered by calling the Nanny agency you reference. But if you're not paying a LOT, don't expect to get good care. And then, I would put a camera in your home so you can see what's going on.