Will your climate let you have Bougainvillea? Google ~hot sun deck wall site:gardenweb.com~ for several similar discussions here on GW.

Make sure you use a stain brush and not a paint roller to apply it. Paint roller doesn't penetrate wood deep enough. That's number one cause of peeling deck stain. It's always better not to paint in direct sunlight, but I am not sure how hot it gets where you live.

A phony plant in a sun room makes no sense at all and if you are actually planning on using the space invest in some solar shades and since the space gets so much light I would choose another color instead of yellow.The reason you see so much wicker in sun rooms is that the sun ruins fabric and wood very quickly that is why I say solar shades to keep the view but block heat and UV rays.

if the top room is being built, then have the 'sunroom' included into the plans. You need the structure to support the room above. Who has drawn up plans for this and why wasn't the first story included? obviously it's not going to support anything as is. but if all you want are ideas for the window, why not do a bifold door across the span? not cheap, but it would be nice for a walk out patio. or sliders or do a couple of french doors and tall windows