Normally the underlay should not be fitted on the riser at all and on the tread it should be taken in from the edge if the carpet about 3-5cm. The fitters should have known this. I think if you are not happy with the job there's not reason why you should have to live with it, the fitters should come back and do it right. It's not normal to see the underlay from the side.

About osmo on stairs: if you put too much on, it will be slippy. Used sparingly it will be fine; it’s easily touched up. There’s tester sachets available for all their colours, and you can fix it if you make a mistake—I’ve done it, although granted you can’t get it back to a normal raw surface, I don’t think. As far as maintenance goes, I had it on my upstairs hall in my old house—including outside the bathroom. I found it easy to clean and look after.

Entirely depends on the stairs and what's under there. We live in a really old cottage and have a dog leg staircase with a half landing like that, which is also in the kitchen. On the edge of ours is a supporting wall for the landing. ( I'm a little intrigued as to what's holding yours up. ) Here' it is when i was renovating last year. I think, that if you ever decide to have a new kitchen, then i'd work with the space as it is, completely change the layout and design so that you work with it instead of against it, & build it in to the design. For now, i'd just go with what's been suggested above.

That’s very helpful - thank you. Seems painting existing is the most popular which is what I was leaning to. Just have to decide colours which is another dilemma if anyone wants to make suggestions. Walls are Timeless.