Is the onah beinonit always the 30th and the 31st of the cycle?
Technically, the onah benonis is the 30th day. The reason for checking etc. on the 31st day is essentially for two reasons, first of all because very often the 31st day is the yom hachodesh- the same day of the lunar month as the beginning of he period. Secondly, because according to some poskim the onah benonis is actually on the 31st day.
Y:D 189-1, Shach ibid-1, Igros Moshe Y:D 3-50.