@: Thank you in BIG LETTERS for your kind and patient assistance with this. Your last couple of tips in a PM got it working!
As you mentioned, the two remaining problems were simply:
• That Multiply(,-1) node I was asking about? I had a silly typo and it was doing Multiply(,1). It’s funny, if this were code in any programming language, I have to think I would have noticed an expression like (foo*1) and wondered why I was multiplying by 1! 
But in the “blueprint-style” material editor I didn’t notice it.
• I had the BunnyMaterial hooked into the BunnyParticles the wrong way. Instead of being attached to the particle system as a whole, it needs to be connected to the emitter’s Required tab.
With those two changes, it works!
I don’t know how to thank enough, both for creating this very clever technique and for the assistance here and in PMs.