We'll have to ask USMC, but I don't think anybody calls it an "Ef-Fay-A-Teen." They either call it an F-18, or a Hornet. No one calls the F-22 an Ef-Fay..they call it an F-22 or a Raptor.
Only if you are trying to be as correct as possible. For instance at an airshow I'll often answer the question -"What kind of airplane is this?" with the full name. It will often lead to a follow-on discussion about why it has such a different name and so on.
Other times I would say that you are right - F-18 or 'Hornet' are the popular shortcut names. Never been called the A-18 that I know of
It has been used in the past but some very ground minded Marine CO's (think former A-6 background) in Marine Ready Rooms as the f/A-18 (emphasis on the Attack role) which is fairly accurate, but it really was meant to put emphasis on the training percentage spent on either role for instance.
I thought the name Viper came from the USAF guys who hated the official name 'Fighting Falcon' and that 'Battlestar Galatica' (the TV show) was popular at the time and they like the little fighters in the show which were called 'Vipers' so it became the unofficial name - and I agree, I don't think I've ever heard an AF guy called the plane anyting but a 'Viper'.
USMCFLYR