yitai- The food itself may reach the temperature of “bishul’ but it is absolutely not in touch with anything else,certainly not the walls-especially if the food is covered (as it should be) No food ever touches the walls and I do not believe that the walls ever reach a temperature even near ‘bishul’.
hello99- I am not sure how ‘zeli aish’ helps you here. Basically, the argument about microwaves is ,very simply, whether you can use the same oven for both fleishig and milchig (leave pessach aside for now) Especially if you cover the food, I cannot see any reason why not to use it for that purpose.