I read I am only able to receive on multiple MIDI channels while in Song or Pattern / multitimbre mode but not voice or performance which are single-timbre.

That is correct and means to accomplish your goal you will need to be in Song or Pattern Mixing mode to setup your configuration. The Parts in a Song/Pattern MIXING can be set to any MIDI channel. Set all the Parts you want to play from the Motif ES keys to Receive Channel 1, for example, and all the Parts you wish to trigger with your pedal board on Channel 2.
When you *select* Part 1 - the Motif ES Keyboard will play those Parts set to Rcv Ch 1;
Set your pedal board to transmit on Channel 2.
Remember to set MIDI I/O = MIDI and connect MIDI Out of the pedal board to MIDI In on the ES back panel.

From the Song/Pattern mode
Press [MIXING]
Press [EDIT]
Press

to view Part 1 parameters
Press [F1] VOICE
Press [SF2] MODE
Here you have access to the Part “Rcv Ch” (Receive Channel)

How can I perform keeping both the Motif ES keyboard and external MIDI pedals on the same channel?

If you place all Parts on the same MIDI Channel (like in Performance Mode) then you would have to use NOTE LIMIT to set *split points*. For example, if using the pedal board for bass then set the Note Limit regions so that the Parts all sound below a certain point, say F2. Then set all Parts you want to respond to the ES keys to notes F#2 - G8.

From the Song/Pattern mode
Press [MIXING]
Press [EDIT]
Press

to view Part 1 parameters
Press [F1] VOICE
Press [SF3] LIMIT

Would I have to set up specific note/pitch limits for what the pedalboard transmits vs what the Motif ES transmits on the same channel?

Correct-a-mundo!

So, in Performance mode, whenever the range of two or more parts overlap, all connected MIDI controllers, in my case the keyboard and the pedalboard, will trigger and play at the same time, all such common notes within a shared range? There is nothing to be gained by creating Master Mode Zones, correct?

There is nothing to be gained by creating Master Mode Zones, correct?

Master mode, as the name implies, is designed for when you are using the Motif ES as a master keyboard controller — means it would be controlling other devices, since that is NOT what you’re doing. You are using some external device to control the Motif ES, correct?

Correct, I didn't think zones would help, but I never worked in Master mode, read your article about it and wanted to ask to make sure - designed for transmitting MIDI Out to different devices not in.. So the only way to give two MIDI controllers free independent reign over the whole Motif ES keyboard without them triggering each other's voices would be separate channels in mutli-timbre / pattern and voice mode.

Pattern Mixing or Song Mixing mode.