What I’m getting at is: if jumping out of airplanes is easy and if Cooper is experienced, then why ask for a reserve?
One could label some things Cooper did as disguise:
1) sunglasses
2) smoking
3) saying "front" and "back" chutes like a whuffo.
4) Wearing a suit.
5) timing things for a night jump
6) timing things for a thanksgiving jump?
If he had clearly announced what he wanted and where to get them, that would have been a pretty strong tipoff he was experienced.
Look at it this way: only dumb whuffo FBI would not see thru all the obvious attempts at disguise.
An expert doesn't need much to jump the 727. Making himself look inexperienced takes advantage of his experience, to create confusion for his pursuers.
I mean, he picked the night jump. He picked the time he knew he'd be jumping at ...evening.
Now things that really did matter, he gave exact instructions on
1) flaps
2) speed
3) altitude
so he betrayed some knowledge there, because it was critical to his success. The other stuff wasn't critical. (in terms of how authorities responded)
That sounds like good planning. He knew he could handle a night jump, but his pursuers would have a hard time finding him at night.