1)
In addition to Mike-san's explanation, the topic of the first sentence is "he", and the topic is not changed in the second oneyet, so "彼はもう誰も信じなくなって" is the most common interpretation.
2)
If the speaker is talking about his Easter "I wish I have a good Easter", that's potential.
3)
I think the former interpretation is correct. A general description for "my drawing picture" is 彼は絵を完成するのを手伝ってくれた。. 彼が(third person の)絵を完成するのを手伝った。 can be possible, but 絵を完成するのを手伝ったのは彼だ is more common for the case since 彼 is the key information the speaker wants to convey.