They may have used small dowl rods or biscuits to join the chair. Which usually means a void/rebate is created and then a proud piece glued into the void. (Think male/female sockets etc). So you may be able to gently tap (rubber mallet, not metal hammer) to get the pieces apart. Use fabric or thick wadding or sponge to protect your timber when you whack it.

And the seating areas are different, don't try to make it look like one area. I would put minimal ( if any) teal around the black sofa. Do your dining room. Then give you living room the same respect, it is it's own space

The chairs are a good match. Leather is probably best, great for kids' spills and accidents, but any fabric will do. Not sure on purple with green - those two colours together clash a bit for me. The green kitchen looks great, and I'm a fan of dark wood. The only thing throwing it off at the moment is the yellowish coloured walls. If they were repainted a soft white, this would make the green and the dark wood really pop, giving you a fresh and sophisticated, calm look which I think would work well. You could then upholster in a green to match the kitchen which would look appealing. Finish it off by some fresh white flowers as accents in the room to draw the eye and you've got a beautifully transformed space with a fabulous feature dining area :)

Oops sorry in wrong section and I don't know how to delete