Good questions and answers.
I sometimes think about Lennon and McCartney, It was a real give an take yet one person still had a core idea and the other one would help out with the tweaking...yet since they were there in the room with each other, both could play, sing, etc. that made for a powerful song team.
As I read lyric only, I really wonder, how will that work once you add in music? I know I change my lyrics based on the music and then tweak it as needed. How can folk in separate physical places bring the two , the lyric and music together and create a good song.
Yet to do all that by text alone? I have a hard time seeing that work out without a lot of extra time and work involved.
I just did a collaboration with a female singer / song writer. I was fun and cool, but still very hard. It was a first time collaboration and though it went well it had problems with the final tweaking and she is right here in a nearby town. BTW, we did get a recording done of the song, still needs a bit of work.
One of the problems is that is just takes a while to get used to working with a co-writer. There were changes I wanted to make and did, but I didn't run them by the co author first....this was just 'cause I am not used to having a partner in creating a song. Yet I just said sorry and let go of some of the changes that I thought improved it...and it still was a good song.
So collaborating, I think, is a very tricky deal. Seems like it would take a long time to get that synergy going that a good writing team needs