Cecil wrote:
"But when I ask what happens to the energy in these two cancelled waves,
all I get is silence."

It`s golden!

Cecil then wrote:
"Maxwell`s equations tell us that all the energy in a Zo-matched system
winds up incident upon a load."

Would Maxwell lie to you?

The question must be hidden between the lines. It may be: What mechanism
reverses the wave?

Terman may satisfy the questionn, whatever it is. Terman says of the
incident wave:
"---everywhere on the line Eprime/Iprime=Zo.
Terman says of the reflected wave:
"---everywhere on the line Edouble prime/ Idouble prime= -Zo.

The difference is only the minus sign which indicates the reversed
travel direction of the reflected wave.

Terman says on a line with an open-circuited load, that at the load,
voltages of the incident and reflected waves have the same phase but the
current of the reflected wave has the opposite phase from the reflected
voltage.

For a transmission line with a short-circuited load, behavior is the
opposite. Incident and reflected voltages are out of phase but the
currents are in phase. But, there is a 180-degree phase difference
between volts and amps in the reflected wave as in the open-circuit line
case.

The point is that at a discontinuity, upon reflection the phase between
voltage and current in a wave is reversed. That is, that either the
phase of the volts or amps flips upon reflection, not both.

My assumption is that were the phase of both volts and amps reversed at
the same time and place, you would see the same wave traveling in the
same direction but delayed or advanced by 180-degrees. Its travel
direction is unchanged.

Which is cause and which is effect? Often what is cause and what is
effect can be interchanged. Volts across a resistor produce a current.
Current in a resistor produces a voltage drop. Take your pick of cause
or effect.

A reflection may be caused by a phase reversal between voltage and
current. What causes a phase reversal? It`s the discontinuity. Stick a
mirror that obstructs the path of a light beam in the path and you have
a discontinuity. In electrical circuits we should remember Lenz and his
immutable law among obstructions.

Best regards, Richard Harrison, KB5WZI