Thanks for the input. However, I don't think I have a battery issue. But I did investigate it further. It did turn out that from mostly sitting the past couple months (only running every couple weeks) that the battery voltage was low (11.7V). So I had the battery tested (331 CCA) and then recharged and it was 12.5V and 336 CCA. That should be enough to crank the engine in warm weather. That said, I questioned whether the battery was really performing the way it should based on the fact that the battery tester was reading "good". I ended up getting a new (Jan 2016) Autocraft Gold 730CCA under warranty. Voltage was 12.6V on my voltmeter (not sure whether the store tested for CCA). Same no crank result, so I am pretty convinced that it is not the battery - though I may have the store test the CCAs just to make sure.

Here is something else that I may investigate... I just noticed my ignition switch assembly may be a little loose. I've never done the sticky key mod though I plan to eventually - that or just replace the offending part with the MB part #A2024600904. What I am thinking is that maybe I have a bad connection between the ignition switch, the park/brake (autostick) interlock, and PCM. Since the ignition switch output must go through the PCM, is it possible that something related to the ignition switch is the source of problem(s), i.e., would a bad park/brake interlock data going to the PCM result in the PCM disallowing starter cranking, disabling shifting out of park, triggering all the brake and stability control MILs, and triggering a P0600?