To be honest I was a little confused by the formulation of the question and the form of the giving solution.
Also running the challenge took a long time, but I fixed it with putting the given solution table into a container and switching it off.
Not 100%, but I couldn't find out why the multi-row formula tool doesn't perform as it seems to be expected from the challenge. Maybe something changed since then? AMP? Does anyone know? On the answer, when the original value was 'null', the tool seems to be taking the closest valid value. Note, this is different than setting the "Values for ROWS that don't exist". Anyway.
Tough challenge for me but good practice for one particular tool. (Details in spoiler)