It seems to me that you have some important constraints there already in terms of the raised hearth and the width of the brick. How high do you want your artwork to go? How much difference do you want between the height of the mantel and the height of the wainscoting? You might just get some painter's tape and newspaper and start blocking out some options so you can get a visual in terms of height and how it might look....that may help you decide. But I'm presuming you are not DIY'ing the project...a good trim carpenter will be able to help you decide. When I worked with mine, we laid out the design on his work table with scraps of molding so we could get a nice visual of how high the mantel would need to be and how far out it would stick once you stacked up all the moldings...and in my case, how much the bookcases would need to be covered on the edges for the molding to die into.

Ditto what others have said about figuring out your living room layout and where you want your TV before deciding on fireplace. Here is ours.. Fireplace NOT centered on the living wall, but centered with the island. Not because I cared about the island in relation to it at all but it just worked out that way as I did not want a TV over the fireplace. I wanted our TV to the right of the fireplace. I can tell you living in the space, having a not centered fireplace does not bother me one bit and the way it's laid out makes complete sense. We also debated putting our fireplace on the bottom wall in the picture, but that's an interior wall and it would have been more difficult to vent and my DH wanted a straight shot at the TV from the kitchen since he's the cook and the watcher of all sports.

I looked at your ideabooks. You are going about this the right way. You might want to consider starting an idea book for your three seasons porch. Bear in mind that there are many different names for a “three seasons porch”, such as sun room, solarium, all seasons room, and Florida room. using different names might get you more results in your search.

how fun to be doing a refresh of your room. I am not a professional so I dont know the regulations and code for fireplaces, so i cant answer those types of questions. I do think though that I would keep your fireplace surround very simple. I say that because you also have to work with the 3 windows on the right and right now the fireplace seems to not quite work with those windows. I think part of it is the color, i would keep the wall color consistent with the rest of the room. the other idea is to just tile that corner and feasibly not have a mantel or hearth, just keep it simple. heres a few ideas. I will also say the way you approach the fireplace depends on the feel you want for the room in total. heres the concepts I was trying to explain