It is not equivalent to the accessory position. C-Maxes with a key actually have a third setting, Accessory, which basically only powers the radio/MFT. It does not power the other displays on the dash, the climate control, etc. It then has an On position, which is similar to pressing the start/stop button without pushing the brake pedal -- this powers all the electrical systems but, as you note, does not provide propulsion. The key then has a third position, Start, which is like pushing the Start/Stop button with the brake depressed.
As I understand the difference, Accessory does not connect the HVB, it runs solely off the 12V battery -- though it does have a lower draw since it is only powering the radio. By contrast, when "On" (without going to start, with the key, or holding the brake) the HVB is connected, just that it cannot be recharged by the ICE, but it will continue to recharge the 12V battery until the HVB battery's power state gets too low. How long the battery will last depends on the draw of the car and the charge level of the HVB, but since all systems are on -- radio, climate, all computers, etc. -- there is a much greater draw on the batteries.
While I understand you might feel this is semantics, it can cause issues for some activities with an SEL. For example, if you want to add an address into your Nav unit, with keyless entry the car must be "On" to do this; with an SE you can put it in accessory and not have everything on. Another example is going to a drive in movie, where the sound is transmitted over FM radio; with an SE you can put it in accessory and easily play it through the car radio, with the SEL you will have difficulty using the car radio (since the car will be "on", including all the dash lights, even if it can't be driven).
No problem here. I see your point on the difference. What you can do is turn it on, then turn it off. With the doors closed that gives you 10 minutes of power for the radio.
Some people (including dealers) seem to forget that the C-Max has to keep some power running at all times. To keep the door sensors for the keyless entry enabled, for example. It isn't much power, but the car does need to be driven periodically. I recall the owners' manual indicating once every three days or so, but I haven't read the manual lately (and I drive mine every day).