Excellent questions!
You can work out all the steps for a project, but you only need to list the next one. The Next Actions lists should contain work that you can do now, so any dependent actions should stay off it (unless you're handling it electronically and have a way to automatically hide those).
Multiple parallel actions can be put on your NA lists, but there is a concern here. It's easy to fill up a list with tons of Actions, and be overwhelmed with a huge list. Better to be a bit more judicious, and somewhat limit the number of Actions per Project that you have open at once on a Project.
(In other words, if you could honestly do any of 10 actions next on a given Project, all other things being equal it's arguably better to make maybe 3 of those Next Actions. But it's a judgment call.)