What does very narrow mean? Residential stair codes say the stairs from side to side must be a minimum of 36", but when handrails are added to both sides the minimum width between them is 27". Measure the open distance in your stairwell.

Look for accent or side chairs. Those with narrow arms are most likely to fit through the stairwell.

The size of a chair should be listed on its tag, but take a tape measure with you to be certain. If you want a chaise, consider using an ottoman with a chair.

I find chairs to be the most versatile furniture for small rooms or those with nooks. Sectionals are the least versatile. They tend to only fit one way. I recommend you read this Houzz Article on the subject. You might be able to find a small loveseat or settee, but they can range from 48" to 72" with most being closer to 60". However, if your stair has no turn the length of a piece is less of an issue. Below are three arrangements of chairs.

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Always try out furniture in the store to see how comfortable it is. Seriously, take a book and read a chapter in a chair you like. Ask about the quality of cushions. A denser foam will last longer. HERE is one article on that.

Look into apartment sized furniture. City dwellers in multi-storied old buildings have dealt with this conundrum for generations. Prime your walls white first, and then consider painting the big square column and the ceiling of the gable end with the window in a rich color, keeping all the walls and the ceiling around the column white.

Are you replacing that window to be legal egress window?

I like the inspiration photo but also love areas like this wallpapered all over

Put the attic stairway rail on only one side, but do that AFTER you move your furniture pieces up into the attic. You should be required to include an emergency escape window up there.

Is that attic actually approved for living space not all attics are? Those low walls look a lot lower than 6' . This is our attic bedroom in our old house it was the only space for an office I would not do an accent wall especially in an office space since those spaces are already busy.

I like your inspiration photo with the window being the accent wall. The suggestion of chairs for seating makes sense and many chair styles are more inviting than most couches. I wouldn’t use apartment modular stuff up there, but rather treat it like an escape room. Cozy throws and footstools with cushioned chairs sound like a place I would want to go to with my morning coffee.