Regarding animation, you cannot get EXACT frames. As the frame rate fluctuates, there always will be interpolation of some sort. The larger the animation and the more keyframes you have, the more accurate it will be. However on practice, the difference is minimal and I cannot image a case where what UE4 has to offer in this respect would not suffice.
As for custom render passes, you are limited to what post process has to offer you. You can modify that, but that would require overhauling rendering pipeline and is not available by default.