I think too much theory on this subject can confuse you.
If you see that you have no energy to train whole abs and obliques on the same day just do it in different days. Next time change the tactics.

I prefer to make supersets on abs and I always change methodics here. Some time I just train abs at my home in the morning on empty stomach, it helps me feel better my abs.

My favorite exercise is Hanging Leg-Hip Raise. It works very good for lower part of abs.