Packaging, I've got worked out. Actual sourcing of chemistry, somebody else is going to have to work that out.
Package it in paper juice boxes. Temper the sealed boxes in hot water. Tear open, mix and pour at the right times. It's all premeasured.

Mobius loop: Put a notch in the edge of the Mylar leader. A cam and follower rides on the edge. When it hits the notch, it trips a mechanism which rings a bell, advances a counter or sets off an automated process on the microprocessor. (If a microprocessor is used.)

If you can't use a cam and follower, strips of metal foil tape could be stuck on the film with adhesive. They move past a detector, the same way that is done on cinema projectors. (It's basically a miniturized metal detector.) The detector lights a light, rings an alarm or advances a counter.

I prefer a cam and follower riding over a notch. That provides a positive stop at the right point in the process which lets you know the film is in position.

BTW: I didn't pay attention. You don't have to re-expose a third time for the third reversal, do you?
That makes it simpler. Only two laps around the track are needed instead of three. Only a red/blue filter is needed instead of white.
I should have paid closer attention before going off on a tear.