Kohler makes sinks in about 20 different colors. They can be special ordered from most any big box or plumbing showroom and then any granite fabricator can mount them to whatever slab of granite that you choose. Dark colors like black or cobalt show water spotting horribly. The medium colors like Mexican sand are a good compromise between white and a dark color, but then I think of all of those bathrooms out there that thought that pink or yellow fixtures would be a good idea at the time. THe light colors that area almost white like the biscuit, bone, or green tea would have the longest staying power and eventual matchability and still not be white. But, there's a LOT to be said in favor of classic white.

The sink will be bearing on the cabinet along the whole back and several points on the other three sides. Put a bead of silicone in these areas and you will have no problems. Gravity is a reliable downward force.

Seems to me that with new counter & vessel sink the only thing significantly raised is the bottom of the sink. You’ll also have to do some plumbing modifications to add under sink piping & of course now there’s a problem with either where backsplash was with the old counter or with the mirror which starts at backsplash height. Of course you’ll also need new faucets. It’ll be like pulling a thread. Do you like vessel sinks? They‘re decorative & generally a lot more costly than a bathroom sink which you can get for less than $100. If you already know it’s hard to find a 40” counter (I don’t know that), then how about get a new counter, you can get a quartz or granite remnant from a stone fabricator, get an undermount sink to maximize counter space, and have the vanity built up from the bottom. I know that can be done - a frame built to raise the vanity. Check out having vanity professionally refinished to clean up the oak - or professionally painted if you want to change color. Can’t say how much this will cost but how long will you be in this house & how much inconvenience is the too low vanity for the people using it every day? That will tell you if it’s worth it. It’s also a value add improvement to house.

Interesting place. Never saw fireplace wall facing a bathroom-but I wouldn't cover the stone. You liked it enough -other people might like it enough. Also not enough info about the house as a whole to know whether shiplap would work, aesthetically. If you like it-it seems you have enough to do already, with new flooring, vanity, fixtures, etc. I'd choose a different color for the walls, even if going dark-but if you love yours, enjoy it, and repaint lighter neutral only when you're about to list. Or maybe a different vanity and floors will prompt you to choose another color. In short do big remodel only if you think you yourself will love it. Don't renovate for other people who might or might not like it in 5 years. If you always look at the slate and wonder why they chose it and how you'd do something different-then it's a good time to redo. Usually when things are harmonious, others will appreciate it as well. As for the spa feel-oh well..to me, it's overrated. If I want to be in a spa I'd go to a spa. I don't want my house to remind a hotel (unless it's Kit Kemp's one or something..but her rooms are great precisely because they don't look like hotel ones..a lot of thought and individialization went into each one)..and I don't particularly want my bathroom to be like a spa. Mileages might vary and all that, obviously. Would be cool to see the house, to get a better sense of things.