What rubbish! Soapstone is very dense, and very stable. Fluctuations in temperature are unlikely to affect it in the least. Soapstone is often used in masonry stoves because of it's ability to store heat and its very stable nature. If anything were likely to expand and contract around a countertop, it might be the wall or the cabinetry, but even then, changing humidity would more likely be the reason. That being said, a very small gap between the back edge of the countertop and the wall isn't anything to worry about, unless you plan countertops with no backsplash. If you're installing backsplash above the countertop, the small gap will be covered up.

I'm almost finished with a kitchen remodel due to a water leak. I got new cabinets and appliances, the granite was reused. I didn't care whether or not I kept the same backsplash but the insurance said it wasn't necessary to have it replaced. So when they installed my new cabinets, to make them level, they were running higher (above the bottom of the lowest tile) so they took off the lowest row of tile. They explained that once the granite was reinstalled, it wouldn't look right to put something back in that area because you'd see how it runs uphill (or downhill) so we tore off the entire backsplash. I just had the cheap white 3"x6" subway tile from lowes, and I like it so that's what I'm having done again. But I'm soooooo glad that we tore it all out and re-tiling the backsplash. It will look so much better. It's getting done tomorrow. I personally would rather have the backsplash redone than to go with the 3" piece. JMHO BTW the insurance paid for a new backsplash since the granite couldn't be installed exactly where it was before and I got $1100.00 for a $300 backsplash :)

No thoughts on thickness as ours is 3cm, but we have Barroca and I would never do that again. It is a very, very soft stone--on ours, we have pitting all over the surface after only two years and you cannot, for instance, set anything with a ridged bottom (olive oil, beer, wine, etc.) down without a coaster or you will get a permanent imprint. The softness may vary somewhat based on the quarry and distributor (ours is two years old and purchased on the West Coast) but test it extensively before committing to it. I really wish we had paid more for a harder stone. (I did find that soapstone was cheaper than most darker granites where we are when we were looking, though, which was a surprise!)

On walnuts: when my son was young he was allergic to peanuts, they gave him hives. One Christmas season I was cracking walnuts while he was sitting on the other side of the peninsula and he began wheezing as I was cracking. The wheezing wasn't new, he had asthma, but the direct cause that time was the walnuts. The good news is that he outgrew his allergies to peanuts, eggs, and corn by the time he was 7. (The egg allergy was keeping him from getting some immunizations, which was a pain for school purposes, so the allergist sent out blood tests and when they came back clear he vaccinated the kid. He made us stay in the office for 3 hours, but all was well.) My point is that I think you're wise to avoid a walnut based product under the circumstances.