1. its used as a last resort because it says in the ayah if u look at it, that he must first advise her. if she doesnt listen, then move beds, then if she doesnt listen u can do it. and allah has said that if a man knows his wife will leave him the instant he touches her in that way, then it would be better that he never tried hitting her. therefore it is a last resort.
2. the quran and hadith do clarify that its only done when its bad. its not direct, but if u read the whole thing, then u will know
3. i agree that a person definition of extremely bad differs. thats why allah says that if a man knows his wife will leave him, then he should never try it. but the thing is, there are some woman - and men - who will remain stubborn. and if she genuinly doesnt do something extremely bad, then its definately his fault.
4. no the quran is not contraditory, u have to read it with the intention of understanding it, not to pick out its faults. becasue if u do, u wont understand why god has allowed some things and not others.