In those two phrases: да ну and ну да, the key point for understanding their meaning is that ну means uncertainty applied on the following thing.
Moreover да in the beginning of a phrase (e.g. да ну) means not agreement but rather emotional emphasis: да что же это! = what the hell is going on!?, да отстань ты = get off me!.
So да ну .. = I have a very strong emotional concerns\uncertainty about ... (may be strongly negative concerns):
_да_ *ну* тебя = I have _huge negative_ *concerns* about you = get off
_да_ *ну* что вы = I'm _very_ *uncertain* about if you should or should not help me = very polite form of saying thank you (typical of 'upper class', even little bit obsolete)
- or just
-Пошли в кино! \ -Да ну! meaning -Let's go to cinema! \ -No I really don't want to! I think that's not goin' worth it!
But in ну да, да means just yes, so ну is applied on yes: ну да = yes, but I'm not 100% sure about it (in some cases even meaning that person says yes meaning no).
But for some strange reason ну might also be an emphasis word :) (usually coupled with уж emphasis) meaning more like 100% this time!, while да-emphasis means more like just I am very emotional. For example,
ну уж нет! = No! (with a very slight not now meaning)
ну уж теперь ты попался = уж теперь ты попался = ну теперь ты попался = now you're caught!
In fact there are plenty of emphasis-words and they also might combine giving totally opposite meaning, so use them wisely :)
The most complicated probably is да нет наверное that is translated word-by-word as yes no maybe, but actually means I'm very uncertain, but I guess the answer is no.
да ну его к чёртуor something stronger. – Nikolay Ershov Apr 04 '15 at 12:15