You’re going to miss content no matter how you play. It’s the nature of the game. Some decisions lock you out of options.

I can’t think of anything you’d inherently miss from couch co-op.

I played through the whole game with my gf and didn’t see nor read anything that would indicate that.

Regardless of if you play, solo or with up to 4 people there are 4 characters in your party. Your main character can be one of the predefined ones or not. You will probably have/want a mix of both.

First play through will take more than 100h and you will still have plenty to see for subsequent playtroughs. It’s literally impossible to see all of it first time regardless of how you play.

Just get it, it’s not just a GOTY, it’s one of the best games in a long while.

No. My girlfriend and I are 140 hours in and still not finished, and I’m amazed at how smooth the coop works with the story. You can each be different places doing different things, or you can travel together, you can each have your own relationships with npcs. A lot of conversations with npcs will repeat depending on who’s talking, but important story ones won’t. As long as you mostly stick together and make choices together, you’ll have every option a single player game does.