Welcome to Yamaha Synth!

I have two questions: is there a way to assign the fast/slow function of the Rotary Speaker effect to the second footswitch?

No. The Foot Switch can be used to Bypass either INSERT EFFECT A or B, It is not available to control parameters within the Effect Type...It cannot switch the Rotary Speaker speed.
A Foot Controller (FC7) plugged into the Assignable Foot Controller jacks, FC1/FC2, can be assigned to control the available parameter within an Insertion Effect.

Here's the details that can help. First, you are always playing from within a PERFORMANCE. So the assignment of Controllers are Performance *Part* parameters. If you recall the Factory "Jazz Organ" Performance, the Insert A is assigned the Rotary Speaker Type. The Rotary Speaker effect has as its assignable parameter SPEED CONTROL...
According to the DATA LIST Booklet "Speed Control" is the realtime controllable parameter for this Effect Type.
Press [EDIT]
Select 02: PART
[ENTER]
Select 05: CONTROLLER
[ENTER]

Here you will see that the MW is set to control "EffA(+)".... this translates to when the Mod Wheel is advanced it will increase the value, in this case from SLOW to FAST. This was selected because the Rotary Speaker's initial Speed is set "SLOW" - had the Speed been set initially "FAST" then you would use MW set to "EffA(-)" - which would move the speed from Fast to Slow.

If you would like to assign this to another controller, you would need to select "EffA(+)" to make the transition.
The FS (Foot Switch) set to "EffA" will turn the INSERT A [BYPASS SWITCH] On/Off, it does not switch the Speed Control. It simply bypasses the Effect.
The FC1 or FC2 sweep pedals can be assigned to switch the Speed Control.
For example, Set FC2 = "EffA(+)" now an FC7 pedal plugged into the Foot Controller 2 jack will control the Speed Control parameter.

Also, some of the voices affect pitch with the mod wheel - is there a way to defeat that or replace with rotary speaker effect?

Again, Voices do not have controller assignments... Those are Performance PART parameters that get applied to Voice you select for the current Part: Main, Layer, or Split
Navigate to the following screen....
Press [EDIT]
Select 02: PART
[ENTER]
Select 01: PLAY MODE
[ENTER]
Here you navigate using the Cursor UP/DOWN arrow, you will find the screen where the MW is assigned to Pitch Modulation (PMod)
Reset the value to 0 if you would rather not apply "vibrato" to the Part via the MW

You can press [EXIT] and navigate to 05: CONTROLLER and assign the MW to control the Insert Effect... as explained above.

Remember to STORE your changes to a PERFORMANCE location. There is no Voice mode, all stored functions are applied and stored in a PERFORMANCE.

Hope that helps.

There is a way you can get what you want (a footswitch to control Leslie speed), though it'll use up one FC jack for that purpose and be of not a lot of use otherwise.

You will need a soldering iron and know how to use it. Or, a friend who does, and a couple beers.

BadMister has already explained how you can use an expression pedal connected to one of the FC jacks to control Leslie speed. You would do that. But, instead of connecting an expression pedal, you can connect a latching footswitch that's been slightly rewired. (You can use a normal footswitch or sustain pedal, slightly rewired, but the Leslie fast would only happen when you're holding the pedal down, which isn't what we usually want.)

Buy a latching footswitch, like the Live Wire LWS21 Universal Latching Footswitch. Also, buy a 1/4" TRS (3-conductor Tip-Ring-Sleeve) plug. Cut the 1/4" TS plug off of the footswitch, and replace it with the new plug. Solder one of the leads to the Tip and solder the other to the Ring. Leave the Sleeve unconnected.

Bingo. You've now created an "expression pedal" that is either at 0 or 127, and switches between the two when you stomp on it. Plug it in to whichever FC jack you programmed and away you go!

I use this trick with a Normally Closed sustain pedal, as an alternate sustain pedal for my CP4. Of course, the simpler fix is to use a Normally Open pedal connected to the other footswitch jack, but this works for me as I don't want to replace a dual sustain pedal that's part of my rig.

Thanks, Bad Mister and Jeff, for the detailed replies!

Bad Mister, I've gotten a lot deeper into the voice and performance architecture since writing the question, and I think I understand now what was happening. When you select a new voice within a performance, it resets a lot of the PART parameters to default values - so the MW-PMod assignment was being turned "on" just by the fact that I had changed the voice. Once I set the PMod value to "0", problem solved.

Jeff, I'm handy with a soldering iron and that's a great idea. Not sure I want to give up the second expression pedal jack just for Leslie control but it's nice to know there's a way to accomplish it. 🙂

I'm still thrilled with the keyboard. It's been a long while since I've had a keyboard that calls me to come play it. 🙂

One last observation - it appears the SPLIT Part of a Performance does not get its own insert effects. I created an organ Performance with a MAIN Part, a LAYER Part and a SPLIT Part. I was able to assign insert effects A and B to the MAIN and LAYER Parts, but not to the SPLIT Part. Can you confirm this is correct?

Thanks for your help!

All three parts have their own A and B insert effects. Please keep trying and let us know.

Tip: There are several shortcuts that can help move around when editing your CP4 Stage.

_ To arrive at the Edit parameters of the Insert A or B Effect, simply press and hold the button labeled [INSERT A], or [INSERT B], respectively.
_ To switch between the three PARTS, while maintaining the view of the Effect parameters, hold [PART SELECT] + the Part you want to view, [MAIN], [LAYER], or [SPLIT]

This will allow you stay viewing the Insert Effect parameter screens yet move between each PART. The left side of the screen will identify exactly which PART you are currently editing.

Each PART can have its own pair of Insertion Effects active within your Performance.
The INSERT EFFECT is active when its button is lit. A simply quick touch toggles the EFFECT ON/OFF, pressing and holding is the "shortcut" to the Edit screen. (This avoids having to fiddle too much when on-stage if you need to get there quickly.)
A PERFORMANCE contains three PARTS, (a Layer and/or Split Part can be stored in the non-active position), each PART capable of being stored with its own 2 Inserts.