But loading Microsoft approved dll like user32.dll and comctl32.dll is possible
Your assumption that comctl32.dll is "approved" is wrong. None of its exported functions is on the list of winapi functions that can be used in a Store app. Its only use is to provide an implementation for the common controls, none of which are usable in a Store app. It can only be used for desktop apps, you cannot write those yourself on Windows RT.
Microsoft had no reason to make comctl32.dll work in the sandbox for a Store app. It is thus expected to fail to work.
Loading kernel32 and user32 is possible, those DLLs do contain approved winapi functions. However, using any function that is not on the approved list will get you a fail in the validation test. You cannot get such an app published.