If a Parsha is skipped on Shabbos Yom Tov, will it be repeated the following week and if yes, will there be 14 Alyiot and 2 Maftir in total?
Also if a special Haftara is read, what happens to the regular Haftara?
Is there ever a case in the full reading cycle where somebody who attended Synagoge service every week would not have heard every word of the written Torah?
The basic law (Megillah 31b) is to read the Torah sequentially, but occasionally the sequence is interrupted (such as on Yom Tov) and then resumed. All other breaking points are entirely customary.
Historically some communities read sequentially to finish the Torah every 3 or so years, and others read to finish every year. The latter one is the widespread custom nowadays. To accomplish this, each community combines or splits various customary sections at various times as necessary into a new single reading for that week (with only seven readers and one haftara) to meet that community's goal.
Accordingly, the only way someone who always attends synagogue would miss any verses is according to some minority opinions that prohibit getting through (or what they call "catching up") too many customary sections at once (see here) for certain hard to understand reasons. But that is a minority opinion applied to a very rare case.
(Note nowadays, nearly everyone follows one of 2 or 3 common sequences of how to combine and split the sections to accomplish this, and ad hoc rearrangements are incredibly rare.)
If a parsha is skipped, it will indeed be read the following week. To achieve this, there are some Shabbatot where two Torah portions are being read (it will happen soon with Matot-Massei). This enables every single Torah portion to be read over the course of a year. Depending on how many holidays fall on Shabbat in a particular year, there will be varying numbers of double readings during that year.
The case of the Haftara is different, if a Haftara is skipped it is not pushed to the next week.
See this article for more details.