would you be dissapointed or happy if it turns out that the nights king is actually someone who will play a major part in the last two books, and martin has just been bullshitting us about him not appearing?

Disappointed. 

While he’s not great at making his deadlines, GRRM’s been very good at being honest with his audience, and carefully parsing his statements when people ask him questions he doesn’t want to answer directly. I think this goes hand-in-hand with his pledge not to change his mind if people guess something ahead of time; you need to respect the fiction, as Griffin McElroy would put it.