Question I:

Number (2) has too many 'what's, and 'gonna' is unacceptable except in slovenly spoken language.

Number (3) would only be useful in limited contexts: 'you never know what happens next until after you've seen the movie'.

(1) and (4) are fine are fine for general use, as is (2a) 'You never know what's going to happen next.'

You can use any of these latter three in essentially the same situations, and no one will notice your choice. The 'what' clauses all refer to the future.


Question II:

It is difficult to use an apostrophe when you are saying anything. In the written language, you can only contract 'killing' to 'killin' ' when you are writing a novel (or otherwise displaying dialogue) and the characters are speaking casually:

'Wha'tchy'all doin'?'
'B'lin' up a cuppa'
'Min' effen I j'in yerz?'
'Naw, mate. Gotta mug?'

It cannot be done in any sort of formal writing-- essays, business letters, or in asking questions here-- but you are free to use the form in personal letters.