Re: multiple 68881/2s, there's actually an Apple knowledge base article floating around (I'm too lazy to look for it right now) that covers the implications of plugging an LC/LCII card with a coprocessor on it into the slot of an LCIII/III+ that has an onboard socket. The short answer is that, yes, both sockets will be mapped to the same location and will conflict with each other if there's also an FPU on the motherboard. The *one* reason, based on the knowledge base article, that you might ever want to us an LC coprocessor card in an LCIII is if you only have a 16Mhz 68881/2. The article says the slot is clocked at 16Mhz even in the faster 25/33mhz machines, so an FPU that won't work at full speed in the onboard socket *can* work in the slot, albeit more slowly.
(And of course, if you actually did that you'd essentially be doing exactly the opposite of using a faster-clocked FPU card with its own crystal in an LC/LCII.)
I'll second that it's amazing just how un-findable photos of a genuine Classic II FPU upgrade are.