"It takes ten days."
とおか and とうか would both be pronounced the same. They're both pronouced とーか, as far as I understand. I'm not sure I've understood your point about spoken language.
- 八 = や
- 十 = とお
八日 is spelt ようか and not よおか because it comes from the modern kana spelling change of やう → よう. (~a + u = ~ō)
十日 is spelt とおか and not とうか because it instead comes from the modern kana spelling change of とを → とお. (~o + wo = ~ō)
I don't think even a single one of these "it [takes|took] __ days" sentences accept kana for the number. I've been reporting all of them since before this course even entered beta back in May!
Numbers are pretty much the only risky thing to type in kana in this course. ^^;
I don't have a clear answer, i do know a lot of natives tend to drop "easily understood" particles like, 「あの子、早いです。」 or sometimes even 「その雲、怖いだね」. This us almost as colloquial as using じゃけん though, so only omit particles when it feels "safe", and never in formal settings without practice