I immediately ran into a mystery. I want to have Part 1 be the drums. But Part 1 is already filled in. I have not been able to figure out how to replace what looks to be like a placeholder with the drum Part that I want. Or, alternatively, I have not been able to figure out how to delete the current Part 1 leaving behind an empty slot so that I can click on the + sign and add the Part that I want.

The secret is knowing where the cursor is... After a few times in front of the screen you will start to find it immediately. You can operate the Montage either using the touch screen and/or by the dedicated navigation buttons.

You can never remove PART 1... That would render the Montage as good as OFF. There is always a PART 1... Place whatever sound you want there but there is never going to be a time when there is not a program in PART 1.

You are starting by going to [CATEGORY SEARCH] and you selected "INIT" > "Init Normal (AWM2)
Go to the "HOME" screen - there are three ways to get there:
_ Touch the HOME icon in the extreme upper left corner of the screen
_ Press the [PERFORMANCE (HOME)] button located just to the right of the Data Dial
_ Press [EXIT]

When you arrive on the HOME screen you will now notice the cursor highlight surrounds the Performance Name
You can move the cursor to highlight the Part Type/Name by touching the Part Name in the screen; manually cursoring to that location, or press the PART SELECT [1/1] button on the right front panel

For now Press the PART SELECT [1/1] button
Press [CATEGORY SEARCH]
Touch "Drum/Perc" Main Category
Touch the Sub Category you desire
Select your Drum Kit
Press ENTER to return HOME

Now you can use the "+" in Part 2 to add your next instrument.
If you wish to DELETE a PART, 2-16, you do so by touching the Part's number TYPE/NAME area
A pop-out overlay appears with "Search", "Edit", "Copy", and "DELETE".

And I have an associated question: are there any Performances containing multiple instruments in the supplied set of Performances? The Motif XF had a ton of these, but so far I haven't found any on the Montage.

Multiple instrument Performances (?) not exactly sure what you are looking for. In Category Search you can set the Attribute to search for MULTIAGENCY. All Multi Part Performances are blue in the search area, Single Part Performances are green. As you will discover Multi Part Perfomances can be to build a single playable sound (like CFX Concert, Seattle Sections), or be multiple instruments with multiple arpeggios (is that what you are looking for?)
The Performances in Montage differ quite a bit from the Performances in Motif XF because...

Press [LIVE SET] Preset > "Best of Montage 1" contains the following Multiple Instrument Performances as examples:
Pearly Gates
DJ Montage
Tectonic Dub

Advance to "Best of Montage 2" using the PAGE Down arrow on your right front panel
T3 Uber funk
Mr. Side Chain
Wave Surfer
EDM PiTCh Bomb
Marine Life
Kreuzberg Funk

In general, you have the eight Scene buttons which can contain different "sections" as they recall different arps and motion sequence combinations. You, of course, can create your own from the supplied arps, motion sequences and Parts. You can add "+" Parts to existing Performances.

Thanks! That was just what I needed. I'm well on my way now