I'm hesitant to respond regarding solid vs stranded internal wire, but this is something I've done some individual testing on in a couple of bookshelf builds, but of even more value I've researched through credible testing and experience of others much more knowledgeable and experienced than me.
I dont think the debate is even worth having. Solid core inductors sound better and perform better than stranded ones. Inside speaker cabinets, solid core wiring should be used exclusively.
The only benefit and reason why stranded gets used for non-internal wiring is due to longevity and durability. The solid core when experiencing repeated manual flexing in external uses, will eventually cause breakage. Even with that understanding there are still professional studio uses where solid stranding is used externally for its improved qualities and just not handled or flexed unnecessarily.
Vibrations or other small flexing from rotating or moving cabinets does not constitute enough movement to ever cause failure of solid core internal wiring. The only internal wiring fails I've ever seen were from people hanging too heavy a wire and the weight of perhaps hanging a 12ga wire or larger on a cheap speaker terminal causes the terminal to break off, but that's mostly in a car audio world where cars bottom out and go over speedbumps. Even in those scenarios I've never seen broken solid core wire, just broken terminals from too much weight.
With weight being the most damaging factor in those xtreme applications (such as car audio, or where someone uses un-necessarily heavy gauge) that's another win for solid core which is more structural than stranded. Side by side, solid vs stranded, solid core is more rigid and somewhat stands-up on its own holding its form without collapsing, resulting on less pull against the terminals, if that was a concern, which it shouldn't be for residential speakers.