Ответ:

1) He built it to cross the sea and fight his rival, Finn Gal.

2) His wife Una is smart enough to get Finn out of the many rocks and hard places.

3) Bendonner saw the size of the "baby" and assumed that Finn was huge and fled back to Scotland, ripping open the dam to avoid Finn's pursuit in Scotland. (есть ещё вторая версия) "Una took the rock, painted it to look like a steak, and gave it to Bendonner, and "baby" (Finn) had a real steak. Bendonner fled when he saw the kid tear up a steak with ease"