If a TP&N supply was feeding a DB that was feeding a mixture of
balanced TP and SP loads the supply cable to the TP DB would be
unbalanced between L1,L2,L3 & N.

It will remain balanced, provided all live conductors (including any active
neutral) go through the same cable.

Take a 3+E cable. Draw off red phase, you get current through the red and
return current through the black. Balanced. You can do the same for any
other phase.

You can draw current through multiple phases at the same time and it will
remain balanced. As the phases are out of phase (strangely), the value of
the current in the neutral doesn't just get bigger as you add current. Some
of the red phase return path may go down the blue/yellow phase instead, for
example.

Indeed, if you draw the same current through each phase, it still remains
balanced, although the neutral current drops to zero! A 3 phase motor will
typically not require the neutral as it draws equally from each phase.

In any case, I see no reason why the electrons going up the 4 conductors
doesn't match the number of electrons coming back, unless they're falling
out of the cable somewhere, or there is a separate conductive path, such as
an earth fault or external neutral.

To be balanced DOESN'T require the same current in all the phase conductors,
just the total current vector in all conductors to add to zero. If it
didn't, a 4P RCD would trip!

Christian.