I don't understand all your questions but here's some. The fuel is used as coolant since it's a better coolant than the oxidizer for a range of reasons. One of

I don't understand all your questions but here's some. The fuel is used as coolant since it's a better coolant than the oxidizer for a range of reasons. One of them is that hot oxygen is very corrosive. Also, because of that a small oxygen leak from the coolant passage to the chamber tends to grow larger with catastrophic consequences. With kerosene the problem is coking. Yes it's more complicated to design a regenerative cooled engine, but existing materials can't take the heat. Some maneuvering thrusters are heat sink designs. Some engines are ablative, with things like evaporating carbon taking the energy. Yes it only needs one engine for landing vs nine for fully laden takeoff.