According to Alltrax's DCX installed in a PDS schematic, to get voltage to the keyswitch, the pedal microswitch has to be closed as well as having either F or R selected.
Apparently that is not true, or you would have noticed it.
I haven't traced out the actual wiring in the DCX to PDS adapter, so the jumper wire between J1 Pin-4 and J4 Pin-4 may actually be between J1 Pins 1 & 4. That would make it work like you described.
(It certainly wouldn't be the first mistake I've found on a schematic.
)
As for connecting the DVGM straight to the battery pack, that's how I've got mine connected and I haven'ty noticed the display getting any dimmer is about 18 months.
I don't know about the others, but the one scottyb sells, draws less than 0.001 amps, which would take over 300 years to drain the pack down to 50% and the self-discharge rate of the batteries if far greater than that.
-------------
Is your stock SoC meter a two wire or three wire?
If it is the two wire type, it may be resetting 100% to whatever the current voltage is every time it is turned off/on.