> I understand that Unicode denotes single characters for both Chinese and Japanese (and Korean outside of Hangul?) even though there are differences between how nations write these 'single' characters, so the result is a Unicode font will look like a Chinese font, or a Japanese font, but not both.

This is "Han Unification", a terrible idea from early in the development of Unicode. The idea was so bad that the affected glyphs are now also given always-Chinese and always-Japanese Unicode points, making it possible to, for example, compare a Chinese character to a Japanese character in the same document.

But the fix exists. You can specify that you have no idea what you're trying to write by coding it as U+76F4 (直). Or you can specify a Chinese character by coding U+FAA8 (直). Or you can specify a Japanese one by coding U+2F940 (直). There isn't actually a reason you'd want U+76F4 - it's just a dead, useless unicode point - but we can observe here that my default font doesn't include a glyph for either U+FAA8 or U+2F940 even though U+FAA8 is by definition identical to U+76FA (since this is a Chinese font).