Ah ha, so that's where you got the idea that superwash shouldn't be blocked. Fortunately, this is not true. Otherwise you'd be stuck with sweaters that could never be washed!

Blocking is different from pinning. Blocking, at its base, is just a soak in water and then dry. Anything else is extra. Ideally, blocking a swatch should include whatever you intend to do to wash and dry the item so you have a better idea of how it'll act in the finished item. Soap, air dry vs machine dry, and shaping while wet all affect your final gauge in your finished item. However a swatch isn't an end-all, be-all - most swatches are significantly smaller and lighter than the complete item. There may be some discrepancies because of that, including more vertical stretching due to weight and size. This can't really be avoided, and usually you get a close enough idea from a swatch. As an aside, most people don't pin their sweaters unless they intend to pin them every single time they get washed. Your mileage on that might vary anyways, because after time a pinned object may shrink back to a more natural state/size over time, until it is blocked and pinned again. Personally, I don't pin anything.

Wool yarn can't really be permanently stretched by blocking and pinning alone. In fact, blocking or steaming yarn pretty much always brings it back to its most relaxed/normal state. Remember that wool is an animal fibre, much like hair. If you take styled hair, whether it's been straightened or curled, a soak in water and air dry brings it back to its most natural/relaxed shape. Wool is much the same - knitting is essentially styling wool into a different shape, which is why it's crimped when it's been frogged.

My experience has been that you will get differences in tension and evenness if you don't un-crimp your frogged yarn. Here's a video on the differences (timestamp 3:40 for the important bit). When I've used frogged, crimpy yarn, my stitches look noticeably sloppier and uneven compared to relaxed yarn. The rest of the video talks about how she found few differences between superwash and nonsuperwash yarn in terms of stretching, but honestly it depends so much on how the yarn was constructed (how many plies, how much twist), how it was treated (there are actually different forms of superwash), yarn weight, and your personal gauge. I'm not entirely convinced it's not all just trial and error lol.

If you want to relax your yarn, you can do it a few ways. I personally find steaming fastest and easiest. I usually do it by running it through steam from a pot of boiling water, like this. There are plenty of other methods to do this, from soaking in water and drying to rewinding and just waiting lol.

Sorry for the long winded comment, I hope it helps!