Finishing of Oriel Window & Overlook Window

[mike2016]

I'm deciding how to finish two areas of my house and wondered if I can use a hardware faced plywood in both cases. Should I add plasterboard into the mix to provide some fire resistance? Or do I layer up with OSB/Plasterboard/Plyood? The Hardwood faced ply may be enough (18mm) and I like the look of wood for the finish of these features when the rest of this space is plasterboarded and painted. Both are in the same space - open plan living/kitchen area. I plan Bamboo Floors throughout. 

 

Oriel Window - Hardware Faced Ply around the inside of the window opening and then a cushion or two to sit when using this space, 1/2 dimmable led lights above for reading

 

Window overlooking living area - Hardware Faced ply around the outside with iron on strips to cover edges and match hardwood. Inside OSB with plasterboard on top, painted, no extra lighting, just landing light. Soft bench to sit on / recline. I plan to install windows on the face and on the left side - no frames, I'll make beading to hold these myself and secure the glass inside them. The other openings will be closed off. 

 

So do I line everything with plasterboard (fireline or equivilent) and then finish or forgo the extra effort? Thanks!