Codes P0302, P0012, P0420, U0073, C1241

When I bought my car I was told that the catalytic converter was not at peak performance and that it may need to be replaced in the future. So I've driven the car about 20,000 miles since then and have kept up with routine maintenance. About every 400-500 miles I have to add a quart of oil. Which it doesn't seem to be burning but I also can't find any signs of it leaking. A couple weeks ago it started sputtering randomly sometimes at 50-60mph and sometimes when accelerating from a stop. Then a few days ago I was attempted to pass a car via a passing lane on a small highway and when I pushed the accelerator the engine lost alot of power and the check engine light started flashing. Also noticed that although it gets over 400 miles on a full tank, lately the gas light starts turning on and off when the Guage shows a quarter tank.
The guy I bought the car from claims it is a catalytic converter issue but judging by the kind of mileage I get I'm thinking that it may be causing the CC to not work properly but the issue causing that I believe is an internal oil leak. Whatever it is I figured by the symptoms it has to be a leak causing a combustion issue but I have no idea where to start.
Also, here are the codes I'm getting from my obd.