1) 그래서 포도밭 울타리 여기저기를 살피며 들어갈 만한 구멍을 찾아보았어요.
The pattern here is verb stem + ㄹ 만하다 (ㄹ 만한 ..as a modifier)
ㄹ 만하다 This means worthy of ... worth the effort, sufficient, ..
He found a hole worth going into.. meaning he found a suitable hole.
집에 먹을 만한 것이 없다 There is nothing to eat in the house. (It doesn't mean there is no food at all, just nothing worth eating.. no desire to eat what is available)
믿을 만한 사람 a person who is worth trusting (a reliable person)
볼만한 영화 a movie worth watching
읽을 만한 책 a book worth reading
아직 쓸 만해요 (It) is still worth using... (it might be broken, old etc) but it is still useful.
만하다 can also be used with nouns to mean the size of.. or to the extent of
noun + 만하다
주먹만한 돌 a rock the size of a fist
2) 정신없이 means without control, with wild abandon
따 먹다 is a combination verb.. 따다 (to pick) and 먹다 to eat.. so it means to pick and eat (by oneself)