Welcome to Yamaha Synth! Forgive the lecture but we give it in hopes it will help.

I have a MPK49 controller and a Yamaha XS Rack module. For example... I have 4 'patches' or whatever people are calling them now.

the "whatever people are calling them now" you will discover is very, very important. It's the same reason your parents gave you a name, to distinguish you from the scores and scores of other children in your neighborhood. What you call something is very important... If I told you to stop the car but failed to distinguish between the pedals on the floor, you might wind up getting scraped off of a wall instead of paused at the stop light. 🙂

What's in a Name?
Now what Yamaha calls "patches" come with specific names. You don't have to like what they are called but for any answer we give you to make sense you will need know what we are talking about. Part of the reason you are here asking the question is you couldn't find it in the manual... And indeed, if you looked up the word "patches" you wouldn't find anything. That term is not used in the Motif-Rack XS, at all.

So first let's try to use the terminology so that you can answer some of your own questions. The words you need to learn to understand your Motif-Rack XS are:
_ VOICE an individual instrument program. Examples, "Full Concert Grand", "Sweet Flute", "Vintage'74", "Power Standard Kit 1" etc. VOICES come in two main categories: Normal and Drum Kits. "Normal" are musical instruments where all notes are used to make the single sound, drum and percussion sounds are assembled into Kits, where each key is a different percussion instrument.
_ MULTI a collection of 16 Voices in a single program. Typically, each Voice is assigned a different MIDI channel so that you can address them separately. The Motif-Rack XS is said to be 16 Part Multi-timbral... in MULTI mode.
_ PART is a set of parameters applied to a Voice when placed in a Multi. Example of Part parameters would be the MIDI Channel assignment, the volume relative to the other Parts, the Pan position, the routing to the outputs, etc.

There are over a thousand individual VOICES in your Motif-Rack XS.
There are 128 MULTI programs in your Motif-Rack XS, each contain 16 PARTS, each PART contains one VOICE.

MIDI 02 : Kick/snare
MIDI 03 : Hats
MIDI 04 : Toms
MIDI 05 : Synth lead... etc...

I would like to map the volume for each of these channels - to a set of sliders on the controller.

See, right away your misunderstanding of the terminology starts you going down the wrong path. First, you want to assign different sounds (you now know are called "VOICES") to different MIDI Channels. For this you need to be in MULTI mode.

Press [MULTI]
The screen and the lighted button will indicate that you are in MULTI mode. The upper left corner of the screen will always indicate the mode.
You can initialize a MULTI for your setup quickly by pressing two button together.
Hold [MULTI] and touch [ENTER]
You have now initialized a MULTI (this assigns VOICE 001 to all 16 Parts, sets all volumes to 100, sets all Pan to center, and each Part to a separate MIDI Channel: Part 1 to Channel 1, Part 2 to Channel 2, etc., etc., etc.)

From here you can EDIT the MULTI to assign a VOICE to each PART as you require.

There are no VOICES named as you listed above. Kicks and snares along with hihats and toms are typical found in a Drum Kit VOICE.
If you want to have separate control over the Kick/Snare separate from the Hihats, separate from the Toms, you could assign three separate drum kit VOICES, to PARTs 1, 2, and 3 of your Multi

Press [EDIT]
Here you can choose to edit "Common" parameters or the individual Part parameters "1-16"
Move the cursor DOWN to highlight PART "1"
Press [ENTER]
You arrive at the "EDIT SELECT" screen... Here is access to all the different Part edit parameters... First lets select a Voice for Part 1

Select "VOICE"
Press [ENTER]
Here you can select the DRUM KIT VOICE you want for PART 1 - it will automatically be assigned Channel 1
set the BANK = PDR (Preset Drums)
Select the drum kit you'd like to use by cursoring DOWN the list of parameters

You can move to PART 2 by simply using the RIGHT CURSOR arrow. The top line of the screen will always indicate exactly where you are:
MULTI mode, You're in EDIT, you are selecting a VOICE for the PART number you are editing... P1-P16 will appear on the right side.

Continue assigning VOICEs to the PARTs as you require 1-16.
Press [EXIT] to return to the EDIT SELECT screen... Find "PLAY MODE"
Here you can set things like Volume, pan, etc. etc.

When completed you can Name and STORE this setup to any of the 128 MULTI locations. This way you can recall it whenever you need it.

As to how to operate your MPK, please consult the manual that came with it.
cc007 will change the VOLUME parameter set for each Part as long as you are transmitting on the MIDI Receive Channel set for the Part.

Either I'm reading it wrong, or my XS Rack data sheet is incorrect. What should be cutoff - seems to map to release etc... not really sure what is going on.

Cutoff is cc74, Release is cc72 these are standard throughout MIDI (at least for the companies who are founding members)
cc71 is Resonance, cc73 is Attack

Hope that helps. Sorry about the lecture, but I've been doing this many years, and I've found musicians who refuse to learn "whatever they are calling it now" continue to have issues because they never know what is going on or what is being discussed. And as soon as they start calling it by its proper names they begin to stop having issues! Yes each manufacturer bends the terms their own way, rightfully so, some times actually there are good reasons... We must deal with this, it's like learning the dialect - same basic language but each has a local dialect!

If you get stuck, post back here.