Answer:
I wish I was a neutron bomb, for once I could go off
he refers to "going off" in the sense of making a big deal and finally being able to show his feelings after a long time of hiding them
I wish I was a sacrifice but somehow still lived on
He wants to be associated with the deep, meaningful feelings of allowing oneself to be sacrificed (perhaps for the sake of a loved one) but somehow be allowed to live on and reap the benefits of that sacrifice
I wish I was a sentimental ornament you hung on
The Christmas tree, I wish I was the star that went on top
He doesn't want to be just a flashy lover whose commitment burns out, or just the slow-and-steady admirer whose love fades into the background -- he wants to be everything, whether that is deemed traditionally possible or not.
I wish I was the evidence, I wish I was the grounds
For 50 million hands upraised and open toward the sky
he just intends to create an image of unity, suggesting that his love is so strong that it would be enough to unite millions of people
I wish I was a sailor with someone who waited for me
To be on the receiving end of that faith, though, would show just how much the other person loved you, and it might even allow your shared love to expand more.
I wish I was as fortunate, as fortunate as me
He may be trying to turn on its head the idea of a singer hoping for a situation better than the current one, or he may simply be making a poignant statement that the situation in which he finds himself is the best possible one, and so that is the one that he wishes for.
I wish I was a messenger and all the news was good
He's probably making reference to the fact that he always wants everything between him and his beloved to be positive, and he doesn't even want to consider the possibility of bringing her pain indirectly
I wish I was the full moon shining off a Camaro's hood
He always hopes to have the perfect finishing touch for the situations he and his beloved find themselves in -- and approach these situations with a sense of fun, daring and freedom.
I wish I was an alien at home behind the sun
He could be making reference to wanting to be unique and wonderfully different, and bringing these values to his relationship.
I wish I was the souvenir you kept your house key on
He could be making a figurative statement about wanting to be in her life as a constant fond memory
I wish I was the pedal brake that you depended on
he wants the woman in his life to be able to depend on him not just in the sense of the more flowery language earlier in the song, but also in the sense that she would use him, in a way he is meant to be
I wish I was the verb 'to trust' and never let you down
He wants to be more than just the things he has already described -- he wants to be the very meaning of the words which she would use to describe him along with trust.
I wish I was a radio song, the one that you turned up
he wants to be the very definition of what she needs in a relationship. The "radio song" reference, though, is obviously a self-referential line to his being a singer and this being a song, and that he would love for his work (