Rod,
Preflight means the inspection you do before the flight. Postflight means the inspection you do after the flight.
Engines have preflight runup procedures, and essentially the same procedures after the flight.
Checking that the magnetos are properly grounded is something done during the mag check, and this can be done prior to shut down; engine manufacturers also recommend a grounding check in which the switch is shut off. Otherwise, how do you know after shutdown that the mags are really grounded?
An idle mixture check is done to clear the engine and determine how close to ideal the idle mixture is set. This should be done at every shutdown.
Postflight inspections may discover all sorts of things. I've found everything from cracks to oil leaks to missing exhaust parts to bird strikes. Post flight inspections allow you to find maintenance discrepancies and get them reported so that they're written up and fixed (hopefully) before the next flight.
Engine manufacturers recommend post-flight engine runs, mag checks, idle mixture checks, and a "dead mag" check for grounding of the magnetos.
The one speed? I knew a pilot who was meticulous in pre flight planning took an eternity running through his checks, had evrything planned to the finest detail. He then ran into a situation in IMC where all his pre planning went out of the window and he was left running on the hoof with no plans and a seriously deteriorating situation. He survived but seriously frightend gave up flying soon after.
Certainly things may happen under very unusual circumstances that mean one can't adhere to standard procedures. This doesn't happen often. Even in single pilot operations, one generally has time to stabilize the situation and then reference the checklist.
This month we lost a generator while operating into Kabul. The generator took out a second generator and our essential bus, which lost left seat instruments, and the cockpit went dark. It was very early morning, and dark, and we were just leaving a hold to fly an ILS. In fact, we were just turning to intercept the localizer. In a location where few alternates existed, and having been made to hold and given delaying vectors, we didn't have extra fuel to go somewhere and hold for a longer period while we worked it out. The essential bus failure has a stabilizing memory procedure, which we did. We then put one generator back on line, isolated the other and ran it isolated, and continued the approach to land. On the ground we ran the checklist.
Kabul has one runway, and a lot of aircraft wanting to use it. Given local factors, one doesn't necessarily want to go somewhere and hold any longer than one must. Also, given that we were already configured, our fuel burn at the lower altitude was high and fuel was going fast. Accordingly, as we hadn't lost anything that would prevent us from landing, and the three procedures would take a long time to work out, we stabilized, landed, and sorted it out on the ground.
We did reference checklists for the stabilizing items in flight, shot through the remaining checklists very quickly, then troubleshot later at our leisure. We'd have been unable to do all that and brief; we had the approach briefed and everything set up before we descended to the hold, and all descent and approach checklists had already been run. The only checklist remaining prior to landing was the landing checklist, which would be delayed until glideslope intercept.
The point is that preparation early can eliminate a lot of the rush and headache of changing conditions. Organization and prior planning can very often simplify the problem by having much of the distractions already squared away.
MC isn't something I'm familiar with Generally in crew airplanes, a multi crew is assumed; single pilot operations in crew aircraft are the rare exception, and one might refer to those as SP (but will generally just say "single pilot" to avoid confusion). I've never heard of anyone refer to standard operations as "MC." Interesting.