Just found this in a list of logic symbols (as in philosophical and mathematical logic):

The statement A ⊕ B is true when either A or B, but not both, are true. A ⊻ B means the same.

I ⊕ U

Either I or you.

You can find the list here.

It comes quite close to what Moriarty says to Sherlock when visiting him at home in ACD's "The Final Problem". And as Moriarty is originally a mathematician it could make sense.
Also, in the tea scene, you can see Sherlock smiling after finding the apple. I think that he got the IOU immediatly. When repeating "I owe you.." later in the lab, he is thinking about the actual words Moriarty said ("...a fall.")

I think you might be absolutely right about cinderella being the third part. In the beginning I was really sceptical, and in certain ways I still am.
But more about this later.
"Aschenputtel" is about a prince trying to find back the girl he danced with at a ball. The princess lost one shoe and now he is travelling the country and asking all girls to try on this one shoe.
And I really think the key to understanding the timeline of the end scene are the shoes.
When Sherlock receives Ms "I am waiting" message in the lab, one can see his flat, soft soles on the table.
Up on the roof, he is wearing shoes with heels and 'capt-toes', meaning an extra layer of leather over the toes. When he steps onto the ledge the first time (ledge number one, without steel edge), still the heeled shoes. When he jumps back onto the roof, when realized he did not need to jump off it immediatly, you see these shoes again.
Later, when he steps back onto the ledge (ledge number two, more flat and with steel edge), he is wearing the soft, plain shoes again.
So, there is definitely no direct connection between the lab scene and the meeting of M and S "afterwards" I would conclude.
Later, we see S standing on the ledge #1 (heeled shoes again), throwing away his phone. He leans back. Cut. No shoes can be seen at the very moment he jumps.
But if you pause exactly at the moment when he jumps, you can see the flat soled shoes again up in the air.

So I think, these are two different scenes. Perhps two jumps, but we can only see one.
I have other video stills underlining this theory (basically containing lots of pictures of finger nails), but I'm not entirely sure, what to make of them.

Problem with the whole theory being, I O U is a riddle from M.for Sherlock. The different shoes belong to the cliffhanger though, the riddle that was given the viewers from the writers...