Can I download a de-esser for the Qu-32 from an online FX library, somewhere?
no
You can download presets for the existing effects which are in the desk but you can’t change the effects which are available.
Change the microphone or educate the person. I’ve never used those on live sound. Not needed with the right gear to start with or changing how person uses the mic.
Just a little narrow band EQ dip in 6-8k area can solve the problem.
Change the microphone or educate the person.
Not really a thing I can do in the middle of a festival. Also not really a thing you can do when the talent is Garrison Keilor, whose sibilance will take your head off.
Yamaha’s de-esser is a very useful tool, and not just to handle sibilance. It’s also great as a deharsher, without killing the high end on softer bits. It’s a dynamic EQ.