1. Ducky has 10 siblings, excluding Spike. 4 swimmers hatched in the first movie, and 7 in the second. There were 8 eggs in the first movie, but 4 of them never hatched.
2. I don't know why Petrie doesn't sing along with the others. Maybe it's his voice, or maybe Petrie has difficulties expressing sadness. I don't remember Petrie ever crying, for example.
3. Because the Gang is a closely bound group, and members of such groups often say "we" when speaking about a subject that is important to the group as a whole, such as taking care of Chomper, a duty which all of the Gang accepted enthusiastically when they wanted to be his "uncles" and "aunts". Although the enthusiasm of others went down when Chomper bit Cera, Littlefoot's became stronger than ever. So, it is not surprising that he thought they all were responsible when Cera turned on Chomper.
4. Maybe.

I don't believe in luck myself.
5. If it were shown, it would be a whole little interesting side story. Spike and Ducky running into bees, then running from them, barely escaping, and then their parents finding them, mending them and giving them a lecture. But maybe it would divert from the main plot, not to mention that every additional minute of footage makes the movie more expensive to make.
6. Spike is a quite reckless fellow.

7. Probably because he was smiling while all the others seemed scared by bad luck. She felt he was disrupting the unity of the group.