Before going further let me state that I do not consider myself to be an expert on anything! (people who consider themsleves to be experts are often just people who have stopped learning).
I agree with most of what you have said Dick, but I'm note sure that total pressure increases in an expansion wave. I think it is just another case of swapping static pressure energy for dynamic pressure (kinetic) energy, with no energy being gained.
I'm also not entirely sure that you cannot get a normal shockwave if the freestream flow is more than M1.
JAR exam questions often ask things like:
If a supersonic airflow is to be compressed by passing it through a shockwave or series of shockwaves, which will produce the least energy loss?
a. Normal
b. Slightly oblique.
c. Highly oblique.
d. Expansion wave.
My initial response to these questions was that supersonic airflows produce only oblique shockwaves so the questions are meaningless. This is probably true (of shockwaves) in most cases, but even an oblique shockwave is normal over a very narrow area at the leading edge of the object that is creating it.
If the object is very blunt then the normal part of the shockwave might be be quite wide.
I believe that these questions are really probing the students' knowledge of the relative magnitudes of the decelerations and heat losses. But even this is a bit problematic. A normal shock wave produces a greater temperature rise and hence greater energy loss, per unit area of shockwave. But oblique shockwaves tend to be more extensive so there is a greater surface area over which to lose energy.
My answer to this type of question is that a series of oblique shock waves followed by a very weak normal one is the most efficient method, but I am quite prepared to be proved wrong.
The original question in this string is also a bit ambiguous, in that it does not state whether we are to consider the total pressure of the airflow going into or coming out of the shockwave.
If we consider an object in flight (rather than an airflow through a duct) a supersonic flow going into an oblique shockwave will clearly have more total pressure than sonic airflow going into a normal one. But a Mach 5 zero degree C airflow going into a normal shock wave will come out at about 1300 degrees C at Mach 0.4, whereas a mach 2 flow going into a 60 degree oblique wave will come out at more than mach 1. The normal shockwave has clearly caused a great deal of kinetic energy to be converted into heat, so the total pressure must have been reduced by quite a bit. So which of the emerging airflows has the greatest total pressure? I suspect that it is the (originally) slower airflow coming out of the oblique wave.