It looks like it was all enclosed at one time. I'd button it back up as designed and have a full size functioning kitchen again. Turning non open concept into open concept is a fad. One of those two half walls is load bearing. You can see the deflection in the long beam.

Another option might simply be to get rid of the bayed portion of the breakfast nook. Square it off level with the two posts on opposite walls. And then, perhaps, construct a bookshelf across the width of the nook, facing the sunroom. You could place a bench for seating on the kitchen side of it. It would eliminate the need for a railing, preserve the idea of a nook for the kitchen, and provide a functional barrier (book case) between the rooms without ruining the view.

As for furniture placement, we aren’t tied to the sofa being there Not making any assumptions here, but you may want to make sure that if you do open it up, you don't really need to put anything here at all. I live in an area of old row houses with the steps down one side of the living room, and many people either have a big sofa or the TV backed up against the stair railing--and the back of the TV is even worse than the sofa. Just because you can and it may make things look more open, think hard about what the implications are of doing so with furniture arrangement and how that looks. To go back to the small houses here, lots of people take out the small entryway and other walls only to realize that once all the walls are gone there is no place to put anything. You can back a chair right up to the vestibule wall that has the front door 6" away from that chair on the other side of a wall, but take the wall down and nobody is going to back a chair up to the front door like that. You can do whatever you want, just make sure that you are really going to get what you want out of that $9000 spent.

The post is lending security to the half wall. Depending on the inside structure of the wall, you may find the wall moves if someone hangs onto the edge of it. Make sure you use Ceiling Flat for the ceiling paint. That sheen currently on the ceiling isn't very attractive. You'll need a surface mounted handrail. I'd select a white ceiling fan without a light for this space.