Howdy
The whole 'thicker walls' concept is a non-issue. Smith and Wesson has been making 357 Magnum revolvers on the K frame for many, many years, and the K frame sized cylinder, with its relatively thin chamber walls, has stood up just fine to 357 Magnum pressures all those years. Even with the thin cross section at the bolt cut.
The issue with K frame sized 357 Magnums was the relief cut on the underside of the forcing cone. This relief cut was necessary to clear the gas collar on the cylinder. But the reduced thickness at the relief cut sometimes lead to a cracked forcing cone with some high velocity 357 ammunition. That was the idea behind the L frame, a larger diameter cylinder, with more clearance between the gas collar and the forcing cone. This meant that no clearance cut was required on the forcing cone, and split forcing cones would no longer be a problem.
It just so happens that the larger diameter cylinder was big enough to add one more chamber, hence the 7 shot 686.
For what its worth, I picked up one of the 7 shot 686 revolvers a couple of years ago. I bought it on a whim, as I did not own any L frame Smiths at that point. I must say, it is not my favorite Smith, but that is because of the ugly full length underlug and the MIM parts. I'm more of a traditional Smith guy.
Just for the fun of it, here is a photo of the backside of three cylinders. Left to right, a K frame Model 19, the L frame 686, and an N frame Model 28. You can see there is not much difference between the amount of metal between chambers of the Model 19 and the Model 686.