I would need a clearer statement of the problem and the context for the
problem - and the notes for the material being taught - before coming up
with a definitive answer.

However, AFAICT, you need information (metadata) about the 'value'
record to identify the key data within the value record.  That hinges on
my assumed interpretation of 'traditional B+Tree', which would have the
index structuure containing key values and pointers to where to find the
records containing the 'value' and the key.

If the metadata isn't available, I don't think there's a way to answer
the question with the given proscriptions.  If the metadata indicates
that the key data isn't available in the 'value' record (for example,
because in this variant of the B+Tree the key data is stored only in the
index nodes and not in the record), then there isn't any reliable
solution at all - other than those proscribed by the question (and even
if they weren't proscribed, they aren't deterministic either).  In this
screwball scenario, you are also, presumably, prevented from having
multiple indexes referring to the same data records (so it probably
doesn't count as the 'traditional B+Tree').

Maybe the point of the exercise was to generate an 'it cannot be done'
response?  Or maybe the point of the exercise was to make it clear that
you need metadata?  Or maybe the point of the exercise is escaping me; I
wasn't in the relevant lessons, so I can't tell what was being taught.