I still have a ways to go before all my content is safely
nestled in P6's own runtime...but honestly Poser opens FASTER

Those two may be related. IME, it's the size of the runtime that matters. People who don't have a lot of content don't notice any slowdown when everything's in the main runtime. People who have huge runtimes (I have something like 60Gb of content installed, not including the stuff that comes with Poser) notice a big difference. I moved everything outside of my Poser folder, and Poser sped up immensely. Not just opening, but in general.

I have something like 20 runtimes on their own hard drive. I am careful to copy the !DAZ folder (and any folder for injectable morphs) to the main runtime.

Aside from that, if a product is packaged correctly, there shouldn't be any problem with finding geometries or textures with multiple runtimes. Unfortunately, a lot of stuff isn't packaged correctly. That's what Correct Reference is for, I guess.