I can't read music either, at least not fluently enough for it to be of any use to me. But I have, in a roundabout way, scored parts for a string section...
The key to Steinbeck's quote is - 'Once the miracle of creation has taken place, the group can build and extend it, but the group never invents anything.'
Certainly this is the way Lennon and McCartney wrote. They never collaborated on an idea and they very rarely did more than help fine tune the other's idea. They were, in fact, more like editors to each other.
The only exception, I believe, is 'A day in the life' which two unfinished song ideas stuck together. No prizes for guessing who wrote which bit.
Jamming to create songs is the musical equivalent of the Hollywood screenplay thinktank, or round table, and we all know what great movies this approach has given us. The only band I can think of that did it successfully (Talking Heads) had an undisputed leader who made the ideas into songs.
Successful songwriting partnerships almost always have strict demarcation of role - one writes the words, one the music. Elton never wrote a lyric, although he did give Bernie pointers, and themes that he would like to sing about - Someone saved my life tonight is a song that was written out of Elton's experiences (accepting his homosexuality, I believe) but Bernie wrote the words. Very good they are too.
All of this leads me to believe that it must have been very tough for Tom and Jack at Sebonack, but Tom's relationship with Urbina seems like a natural sure thing.