Hi, letterer lys here! I thought I might offer a little (trivial) insight after reading your comments on the sfx
In general, you are right that spoken/voiced fx are replaced, along with fx that are contained in bubbles (to look less cluttered/crowded), which is Kodansha style as I understand it. Otherwise, I leave the Japanese sfx intact and subtitle them with a translation in a matching style. But! Your keen eye found a couple exceptions with the "SLAM" and "SCRATCH"! And the obscure reason for that is, the (textless) art files we used did not have any sfx-text present in those odd couple spots (of course, they were still in the printed book), which leads me to guess that those Japanese fx were added later along with the rest of the dialogue and other text, and weren't directly part of the artwork (you can see they're more blocky and font-like than the many other clearly hand-drawn Japanese fx). So the only thing to do was to simply letter them in English. I hope you'll forgive this little quirk of the process
As far as I recall there were only a couple places this happened (I only remember these two, and maybe a few signs, which would be replaced anyway) so it shouldn't be much of an issue going forward.
Thanks for laying out all these details!! I always think it's fun to make and see comparisons between different translations and editions of things, and it's also interesting to hear how folks receive and interact with something I've worked on! (especially when they actually have something to say about the lettering, haha!)