A masonry contractor could take up what you have, prepare a proper base for it and set in cement with mortar between the stones. It would be more like a solid surface then.

The flagstone appears to be in good condition however it seems some pieces may need to be power washed. I would recommend finding a mason to remove the stones, level the walk and patio then reinstall using a polymeric sand between the joints. Polymeric sand is a harder sand that will set the stones, can deter weeds and doesn't track like other sands or stone dust. Setting the stones in mortar is another option but is probably more expensive. You also might want them to reduce the joint size between the stones. Based on the photos, it appears the wall has been constructed as a dry stack wall without mortar. Typically, the stones set in a dry stack application are fitted together tightly which keeps the wall in place. It seems like your stones are loosely stacked which could pose a problem in the future. I would hate for you to pay to replace the patio and have the wall give away in the future.