Potato starch - katakuriko. A kitchen staple for us. Widely used by the Chinese and Koreans, so hopefully that’s something we can continue to buy.
Really hoped to learn more about how milk powder was used - that was the original point to its introduction to Japanese schools; milk powder was one of the food stuffs introduced to Japan at that time, the directive was for the kids to consume it and they (naturally) hated it. Except in Cream Stew. The clock has turned, we’re back at that time (albeit for very different reasons) I could do with some of that old time wisdom, it’s just locked away beyond my reach.

Wasabi - unless you’re able to get yourself the fancy stuff (or grow you own) it’s mostly just standard European style horseradish with colouring. If you can get your hands on powdered “wasabi”, you can mix that with a bit of finely grated daikon (or a peeled salad radish) instead of water to get a slightly more “authentic” texture. Even in Japan, a lot of the cheaper wasabi is just zhuzhed up horseradish.
Even freecia might accept powdered wasabi reconstituted with ground daikon - just don’t disclose what it is! If you add a bit of fresh horseradish too it will be indistinguishable from most store bought brands.