If you convert "ななにち" or "しちにち", you can get a letter "㏦(IDEOGRAPHIC TELEGRAPH SYMBOL FOR DAY SEVEN). Doing "きゅうにち" or "くにち", you can get a letter "㏨(IDEOGRAPHIC TELEGRAPH SYMBOL FOR DAY NINE). When I say the 7th or 9th day of the month, instead of "なのか" or "ここのか", "にち" form is often used. Question is here:
i) You can also use "にち" form rather than "なのか" for the 7th. In this case, are both "ななにち" and "しちにち" used?
ii) You can also use "にち" form rather than "ここのか" for the 9th. In this case, are both "きゅうにち" and "くにち" used?
One more: When we count 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, they will be "じゅう、にじゅう、さんじゅう、よんじゅう、ごじゅう、ろくじゅう、ななじゅう、はちじゅう、きゅうじゅう、ひゃく". Why are the expressions "しじゅう for 40, しちじゅう for 70 and くじゅう for 90" wrong?