Question 1: no canonical answer is available. Logically it can be assumed that B4 was disassembled to make the discovery less suspicious: a complete, intact android being found would raise questions, especially when it's common knowledge that that Soong dismantled all of his prototypes (at least it's common knowledge to people with any experience of Data's history - where Shinzon may have got this information is anyone's guess).
Ultimately, Question 1 highlights one of many inconsistencies in what is a rather poor script with many plot holes. The basic, unhelpful answer is "it was in the script". Sadly, there doesn't seem to be a canonical reason.
Question 2: Kolaran natives attacked them. It says so here on Memory Alpha:
However, just as the searchers find the final piece, they come under
attack from Kolaran natives in vehicles of their own, so Picard drives
the Argo back to the shuttle while Worf mans the phaser cannon.