This confused me here, but I went with the Ableton equivalent (I think) of not sending any data on Port 2 (and assumed the "Set “Mackie Control” = MODX-2 (Port 2)" was a typo).

There was no typo here. MODX Port-1 is for MIDI musical information to/from MODX's internal engine. MODX Port-3 is for MIDI musical information to/from MODX's external 5-pin DIN connector to target external MIDI devices connected through a standard MIDI cable.

MODX Port-2 is not musical (note) data. It's for what Ableton refers to as "Control Surface" data. So you would never use MODX Port-2 as a MIDI track in your project. However, you would use it as a control surface. And Ableton has the option of a standard control surface as MackieControl:

MODX USB Port-1 and Port-3 are two MIDI buses. Each is independent and distinctly different in the target. Not redundant. Port-3 is the exact same as having a USB-to-MIDI cable plugged into a USB port on your computer. It uses MODX's USB TO HOST connection as one end of the MIDI cable and the other end of the MIDI cable are the 5-pin DIN ports on MODX.

Here's a simplified view of just ports 1 and 3 (leaving out port 2):

... note that when "Local Control" = On then your local keys are going straight to the tone generator. When dealing with a DAW - you probably don't want this. You probably want to let the DAW have more control over your tone generator and just use MODX's keys to send MIDI messages to the DAW - and then the DAW can either route the MIDI data back to MODX (in order for the tone generator to sound) - or the DAW can "intercept" and not send to MODX and instead only send to a VSTi - which will have its audio (but not MIDI data) route back to MODX. Or ...

What this means is that you have to deal with the proper routing on Ableton's side for this to work. If you want MODX to work properly as a slave to your DAW as opposed to being directly controlled. If you notice a difference in velocity/dynamic range - then there's something wrong set on your DAW (assuming local control is off). If local control is on and you notice a problem - then the problem might be repeated notes. Both MIDI sent from local control and also a second set of MIDI messages routed from your DAW back to MODX. This can cause phase cancellation since you have two note-on events for the same Parts with only propagation delay through the MIDI path. The net result can be odd sounding (different from how the keyboard sounds when you unplug the USB port from your PC). This would be a MIDI routing issue.

However, one thing I did notice is that the playback volume from recorded audio generated by a MIDI Track is lower than the MODX's audio generated directly from the MIDI Track. RE: recorded sound is not 1:1 to MODX generated sound. How is this corrected so the audio recorded into the Audio Track is 1:1 to sound produced from the MODX by the MIDI Track?

If they are not identical it is likely because of how you are monitoring the signal. When connected to a computer and using an audio interface, you create two pathways for audio to get to the speakers. There is the Direct Monitor path — the way the MODX normally sends audio to your sound system. Then additionally there is longer path way that has the audio generated by the MODX travel to the computer via the USB cable, that audio routed to the audio interface is converted back into analog and sent slightly later than the Direct signal. This is the latent signal. A few milliseconds behind the Direct signal. If you mistakenly have both pathways active during Record... it will sound (funny to the trained ear) but certainly slightly louder as you are listening to two signals simultaneously. Upon playback the original source data is no longer playing so there’s a drop in volume. You solve this by making certain you are not ‘doubling’ — a situation where you are incorrectly monitoring two identical signals.

Doubling can occur when you leave Local Control On, and you echo signal back to the MODX via your MIDI Track.

You only need to setup one path. It is standard operating procedure when recording MIDI to a DAW to work with Local Control = Off. This disconnects the keyboard from the tone generator, and leaves just the MIDI data going Out via USB-MIDI. It arrives in the DAW at the MIDI Track where you Receive the signal In and echo it back Out to trigger the tone generator. If Local Control is On that equals “doubling”

If you are noting a difference between the sound you send to get recorded, and what you playback, it is a sign of a problem.

You should not be able to tell the difference between them, that signifies a well tuned system.
Likewise when you render your MIDI data to Audio Tracks, it should sound exactly like what you sent in, period.