Low production cities should just slowly build the districts that they have the best adjacency bonus for, with rivers a commercial hub, with mountains a campus and so on. Forget trying to boost their hammers just use the hammers they do have as efficiently as possible and build only the essential districts and buildings. Transfer hammers to them by building builders in other cities or buying them, and send internal trade routes bringing hammers and food. They can still be very useful with low production just don't build units there or try for wonders, you have other high production cities specialised to do that (or should have)
Yep. Send a trader to the city so it can form a route to a core city for the +food and +production. As for which districts they build, adjacency bonuses are a consideration, but I tend to use marginal cities to work on districts that I neglect throughout the rest of the empire. Also commercial districts and harbours, just for the +trade routes.