I have a Motif ES with the fabulous PLG150AN board installed.

On my Windows DAW, I have also installed the studio manager,
the ES multi editor, and the AN expert editor. All work fine,
except for the following.

When I put the ES in multi mode, on the device itself, I can
change the effect send levels from the PLG board to the
system chorus and reverb effects. However, since I would
like to store all relevant data within Cubase with each
arrangement, I would like to modify these effect send levels
through the studio manager. However I couldn’t find any
of these in the multi editor or the AN expert editor.

I know I should interpret the ES and the PLG board as
independent synths with their own midi port and software
editors, but find it strange that settings that are present
on the motif itself do not seem to be covered in the
editors? Or did I simply overlook/miss it?

I know I should interpret the ES and the PLG board as
independent synths with their own midi port and software
editors

If you know this then it begs the question, why the question. The PLG boards are external synths… if you want to control their send amounts you address them the same way you would an external synth:
On the Track that is routed to the PLG board use cc091 to set the Reverb Send amount, use cc093 to set the Chorus Send amount.

The Multi-Part Editor address the 16 internal PARTS of the Motif ES., the PLG Editors address… well you know all this…

cc091 and cc093 - why didn’t I think of that…
I never used these for the motif since it’s much more
convenient to use the effect send levels on the multi editor, and
wasn’t even aware that those would work as well…

Thanks! Problem solved :-)

I never used these for the motif since it’s much more
convenient to use the effect send levels on the multi editor

You’ve been using them along… when you use the knobs labeled REVERB and CHORUS, or you set the parameters on the ES mixer, you are actually using cc091 and cc093, respectively.