I'll start off this post by once again saying, nothing is random. If you believe something can happen for absolutely no reason, then what you believe in is called magic. Things can happen for uncontrollable reasons, and those uncontrollable reasons can happen for other uncontrollable reasons, which can happen for other uncontrollable reasons, and this can go on and on. The only argument against this is the creation of the universe.
Anything that cause a variance, such as the air from the jet or anything else will cause the timeline to alter.
The only thing to cause variance is you. Everything else has no reason not to change, and don't tell me "they could change randomly" without telling me what caused that random change, and what caused the cause for the random change, and what caused the cause of the cause of the random change, ect, until you find something that was caused by nothing (i.e. actual randomness).
even a tiny difference due to the nature of the air jet
What caused that difference? Follow the same rules stated in my last paragraph.
if we traveled back in time, who is to say he won't turn his head the other way?
What caused him to turn his head the other way? If it was us or our actions, then okay. If not, use my rules again because human brains don't make different choices under the same circumstances (once again, memories are included in that).
it doesn't make the slightest difference to the fact that a random event can and would happen
What caused that random event? (rules again)
I certainly don't view it as a preset sequence of events.
According to string theory, every possible timeline to every possible universe exists, so it's not exactly that it's preset, it's that everything that could happen, will happen somewhere in some universe in some timeline.
The air jet is given to randomness, randomness by nature
What caused this randomness? (rules again)
duplicated exactly on two occasions
Again, don't say two occasions. It's the same occasion twice, which is still completely the point of traveling into the past.
Variances can occur to change the course of time.
What caused those variances?