Great post Capt O! My dad's MP company was also assigned directly to VI Corps and under whoever the commanding Army was - 5th Army in Italy, 7th army in France. He was always "VI Corps" and would wear that patch, but also either 5th or 7th army patches.
I have all three patches in his footlocker along with the american flag armband that was worn by American forces on the August 15th D-day landing in Southern France. I imagine they wore the armband to be distinguished from the french forces who landed with them. Was that the reason, does anyone know?
Here's the" VI Corps (American) of the 5th Army" Order Of Battle for 1943.
It shows the separate infantry divisions - the 3rd, the 34th, and the 45th (which would of course have their own attached units) and then all the individual units assigned directly
to the Corps - the FA, engineers,quartermaster units, tank destroyer battalions, medical battalion, signal battalions etc. As you can see, there were 2 military police companies,
my Dad's 206th MP Co and 504th MP Battalion's Co "A" that were directly under VI Corps command.
http://www.milhist.net/ordbat/6corpsus.html
What's the highest rank available? that's what we should give Marion! We should get her a uniform with the ranks, insignias, unit designations of all our folks! M1 - Honorary Infantry, Field Artillery, Medic, bomb disposal unit, fighter pilot, military police, anti-aircraft battalion, glider pilot etc etc. Add all the "fruit salad" and she'd be so weighed down, we'd have to tote her around! 
mary ann