Answer 1

Actually it’s an excuse for not doing some thing that’s supposed to be done (usually is not a real or good reason)

Ex : Oh! No! I forgot to do my homework, but I am going to use the power shortage as an alibi to avoid blame and punishment.

This is the meaning in the context of the second chunk of the bookworm, but it has another meanings

Answer 2

yes , i think that too 

but the thing is not always bad and it may be an evidence.

نعم اعتقد ذلك لكنى أعتقد أن الشيء مش ديما لازم يكون سيء ممكن يكون دليل على. حاجة أو عذر مقبول

Answer 3

Alibi means: proof

Whatever it is a bad or good proof