Whoa, this is...extremely...relevant to my interests...adult women, one of them in a position of power as a CEO, and the surprise live-in girlfriend plot thickening...very intrigued to see more. Thank you very much for this, Yuri-ism.
Can't be ntr if there is no love. As far as I can see they are just fuckbuddies.
On page 20, Umi gives the classic "do you want dinner, a bath, or how about me?" line, which is pretty much the most stereotypical welcome-home-honey wifely remark possible in Japanese. That line, all by itself, is more than enough to make clear that they are a couple, and one in a cozily domestic type of relationship.