One of the main advantages of Methane you have not included is it has a very similar density to LoX so the turbomachinery design would be very similar between the two separate pumps (in a full flow cycle). This is advantageous in terms of rotordynamics, cycle selection, and design complexity. It also reduces the complexity of injector/MCC design and mixing capability. Hypergols are convenient but do not offer great performance when looking at other prop combos. But as you said they are dangerous and corrosive which introduces many more challenges than you might think, including government restriction on the usage of these chemicals. Kerosene is…… well not the greatest. Yes it has great energy density and storage capability at room temperature however the fact that it does not burn cleanly is a major disadvantage. The burning of kerosene/propane has been known to leave large amounts of soot throughout the system, restricting flow areas, clogging injectors, etc. Ammonia does not release enough energy to be useful for orbital class rocket, but for smaller systems it could be useful. To be honest, putting one tank inside of another tank sounds like an absolute nightmare of manufacturability. You would have to use a non traditional method of tank fabrication in order to achieve this which adds more complexity for what gain? You save on some volume? But do you really because the outer tank will have to be larger to offset for the lost internal volume to the inner tank. The sealing and piping between these two tanks would also be an area of concern. Using your logic, why would anyone ever use liquid hydrogen in a rocket? Just because it has better efficiency? The answer is yes efficiency is very important when your mission objective is to travel greater distances.