Var and Rev send are only at the part level.

I'm confused as to the routing methods.

Join the club! There are tons of combinations and permutations and the routing diagrams can be misleading.

I'd like some drum keys to be routed to Part FX, some to Performance FX.

That is the simple one so let's start there and use an Init Drum part for reference. Edit the drum part and, as you said, on the 'General / Pitch -> Part Settings' screen the Var Send and Rev Send parameters are at the part level. Even when they are shown, and editable on other screens these two are part level, not drum key level.

Go to the 'Effect -> Routing' screen. This screen is where you set EVERY KEY and how it is routed. Note at the top right is shows Var Send and Rev Send and you can change these. But if you do you will find the new values are also back on the previous part screen. Even though the routing screen is for each key those parameters, and the 'Part Output' parameter, are part level.

The 'Connect' parameter has three options: thru, InsA and InsB. Use 'thru' for keys that should NOT be routed to part fx but that you may want to route to performance FX. Use 'InsA' or 'InsB' for keys that SHOULD be routed to an insertion effect.

The default key is 'C0' and if you spin through all the keys you will find that all of them use InsA for the connect parameter. Notice also that, at the top right you see 'InsRev' and 'InsVar'.

Use 'InsRev' and 'InsVar' to control how much of the signal from the keys using InsA/InsB get sent to the performance FX. If don't want a key to use any performance FX set them both to 0. 

Change the 'Connect' parameter to 'Thru' for 'C0'. Now that key won't use either insert effect but you can see that the top right now shows 'KeyRev' and 'KeyVar'. These now control how much of the key signal goes to the performance FX.

WARNING: if you start with a preset drum part you will have to examine EVERY key in order to be sure that 'InsRev', 'InvVar', 'KeyRev' and 'KeyVar' are set the way you want. 

Drum Part FX are on/off (no send level control is possible) whereas the Performance FX from Drum Parts have a Send Level control - I think. How do I set these up?

Hopefully the above shows where the sends are for the performance FX. There is no insert effect send for each drum key. It either participates in the effect or it doesn't.

 I am also confused as to the purpose of Part Output: Drum. Where does that go? I was unable to find a description of these on YouTube or in an understandable way in the manuals. Can anyone explain?

I can't help you with that specific question since I have a Montage M8X and the M doc has what is likely a significant difference from the Montage classic and Modx doc.

I suggest you wait for someone with a Modx or classic Montage to reply. You can read a new thread I am creating if you want to see how the M is different in that regard.

Hope the above helps some.

Off the cuff, I've seen these routing options but kind of shrugged them off since I wasn't trying to do something different than the other output options I did understand.  I'll fire up the keyboard and maybe search around too in order to better understand what these are for.

Thanks, it's much clearer now. If only the manual were so simple.

For Drum Parts - the "Drum" output selection allows for each drum key to route to a different output.   One use for this is to route different drum kit pieces (keys) to different USB outputs so you can track each key to a different track.  In order to do this, you need to set the drum key's insertion effect connection to "Thru".  In other words, you can't have insertion effects active for this drum key in order to fully utilize the "Drum" output.  Any drum key, when using the "Drum" output, can have insertion effects.  When you are using insertion effects for any drum key with the "Drum" output - the individual drum key w/insertion effects used will be forced to output to Main L&R.  This is why "Main L&R" as the individual drum key output has a black background indicating it cannot be modified.

For any drum key (using output of "Drum" still) that you have the insertion connection to Thru - you can change this individual drum key's output to anything.  Assign L or Assign R (for Montage only - not MODX) or any USB channel or ... you get the idea.   

For MODX the most practical use for this would be to track drums on separate USB channels.  

For Montage this feature has another use - since you can send outputs to either Assign L or Assign R - you could have some outboard effects on the assign outputs and route some drums to one effect and some to another effect.  Or you could send a click track on one drum key out one Assign side and route that to earphones and that click won't go out other outputs.   Or ... you get the idea - Assignable outputs (Montage only) provide a certain amount of additional options.

I did know that drums could be tracked individually on USB channels but I never really bothered with learning the exact mechanism for doing this.