Thank you for posting this! I really think they should add furigana or pronunciations for words like these in the future.
Does "general counter" mean I could substitute it for the specific counter if I ever forget the proper one?
Yes, you're absolutely right. Though I think よんにん is written in hirigana as よにん though it probably doesn't make much of a difference.
Would it be fair to consider this trend similar to ❤❤❤ regular kanji has both an on-yomi and kun-yomi reading?
It's irregular ;w; Hitotsu, futatsu, mittsu, yottsu, itsutsu, mittsu, nanatsu, iforgotwhateightisttsu, kokonotsu, and idk10ttsu someone kill me
Why was the answer to the previous one "Nine PIECES" but this was just "Six" with no word "PIECES" given as an option. Is it a mistake or is it different for each number, or acceptable either way?
It's acceptable either way. It indicates 6 (or 9 or however many) pieces of something, so without context, it makes sense to translate "six pieces".
However, in a normal coversation -where both speaker and listener know the topic- you could just as well say "I would like 6" (六つがほしい).
Sometimes it's a bit frustrating when we get it wrong by using a synonym. I typed in "six units" and it wasn't accepted. But I don't think it's unreasonable to say that "pieces" and "units" are interchangeable terms here. One could argue that "unit" would be more suitable to translate 台(だい) , but I know that counting in Japanese is a very sensitive subject, as they seem to have a different denomination according to the object's shape. Is there any consensus from Japanese-English translators on which English noun should we use for each counting denomination in Japanese?