Its possible, but not likely. If its masonry, the fireplace offers alot of support for the chimney above. Double sided fireplaces have double lintels to support both sides if the fireplace. Also, it might not draft properly, the chimney size is realated to the fireplace opening. If you introduce more air ( a second side ) it could dilute draft and draw poorly. Also do you realy want a double sided fireplace open to a screened in porch? Fireplaces dont offer much R value and that sure would be a cold place to have open to the main living area, not to mention insects and rodents. Glass doors dont seal that well.

If you are handy you could buy bookshelves and cut out of the back a hole where the vent would be. It wouldn't be that difficult to do and you could glue trip around the opening for a finished look. It may be possible to find bookshelves with no "back" on them in which case your vents could still operate. Check Ikea (if you have one by you), they make all different types of shelving. Here is a link that might be useful: Christy's Thrifty Decorating

I agree with bumblebeez, that you may as well leave the wallpaper until you're ready for a proper redo. if you like how the wallpaper looks for now, then that's what matters! I agree with others who say you should keep the fireplace since it's nice to have and it makes a nice divider for a large room. when you are ready to redo the tile, keep it simple-- so, no to patterned tile, stenciling, etc. you already have lots of patterns in your room when it comes to rugs, upholstrey, etc. if you keep the fireplace simple, it gives more flexibility to choose different room accessories if and when you want to change things up. so my suggestion: a white brick veneer for the vertical surfaces, and some kind of dark tile for the flooring around it. you could have a little fun with the floor tile shapes. choose smaller tiles since it's a smaller area.

I love your roman shades and the panels would be completely unnecessary. It would just add another layer of texture/softness/interest to that wall. If you did do something like that, extending the rod further toward the side walls would balance the windows by taking up some of the extra wallspace on the sides of the window. However, leaving just the shades would be just fine. I just feel you don't need anything else on the walls on the sides of the windows. I could see 2 matching poufs or benches (maybe caramel leather) under each window as a nice filler that would add seating or foot rests.