If I stay near Langdong Bus station and from there I take the early bus to Detian waterfall,

Yes, there is a direct bus to the waterfall (about 4 hours). The bus departs at 8:30 from Langdong Bus Station, and the fare is CNY 75. So you will have enough time to visit the waterfall.

Yes, you can take a bus to NN from Yangshuo North Bus Station (about 6 hours). The bus departs at 11:00, and the fare is CNY 130. The bus arrives at Langdong Bus Station. :)