the head monuments are easy to explain... different figures in Engineer society. its the same way the greeks/romans carved different statues of gods and leaders, simple as that (infact, all of the pics of the engineers society in ruins greatly resembles the greeks/romans).
i have a feeling that the scene with Weyland will be a David "dream sequence". It seems to me that he has most likely surpassed his programming and achieved a level of sentience that has never been seen in androids before now. the whole scene seems very "dream-esque" to me. basically it revolves around a "boy" dreaming of his "father" seemingly...