The correct answers are Had I not sneezed, the T-Rex wouldn't be chasing us right now and Had it not been for my sudden sneeze, the T-Rex wouldn't have noticed us.
In inverted third conditionals, the auxiliary verb had moves before the subject. When the sentence is negative, not must immediately follow the subject (Had I not sneezed), making "Had not I sneezed" incorrect.
The first correct option is a beautiful example of a mixed conditional (past unreal cause, present unreal result). The second correct option uses the fixed inverted phrase Had it not been for... to show that a past noun (the sneeze) changed the outcome.
"Were I not to sneeze" is grammatically flawed here because it refers to a hypothetical future or present, which clashes with the past result "wouldn't have noticed."