If you want the seats to swing out from the island be sure to check that the island is substantial enough to take the load, also that the island is well anchored to the floor. If you consider the arm is for example 2 feet long, then sitting on the extende seat will be the same as applying your full weight to a crowbar, two people sitting on one side would destroy a normally constructed island. Similarly, if fitted to a wall make sure that they are anchored well and that the wall structure is suitable. A nice idea to save space though

Out of all the ones you’ve listed I like the one from Perch and Parrow but I’m confused when you say footprint within 48x48cm and this one is listed as 42x42cm. Incidentally I found them cheaper here: https://www.costco.co.uk/Furniture-Home/Dining-Room-Furniture/Chairs-Bar-Stools/Gallery-Palmer-Brown-Faux-Leather-Bar-Stool-2-Pack/p/263151 Also found this one: https://www.wayfair.co.uk/furniture/pdp/selsey-living-hoker-65cm-bar-stool-tcdv2652.html?piid= And a bit of a wild card but the cantilever might make it extra comfy: https://www.argos.co.uk/product/8835576 Also have a browse in the Houzz shop. They have lots of barstools so you may find something there.

I personally prefer the white stools but both are nice and sit well within the room. If you prefer the black and they're sold out why not consider adding a chair at the end? I've only quickly messed around but why not add some accessories to the table? I'm not really a table runner person but there are some nice "scandi" style ones around which would add more texture. It's a nice room, hope you're making use of it :)

Depending on what colour your kitchen hardware is, there a couple of options that would work well in a graphite kitchen. If you have white worktops and a grey kitchen, I would add another colour or texture to the stools to create another layer to the scheme. If you don't like using bold colours, you could use an interesting textured metal or wood for the stools. You could try using something similar to the silver wire stools below for a more industrial style Or wooden stools are always lovely for a more contemporary country look. I would use a lighter wood, to add contrast to the dark grey kitchen and white worktop All the best with your project