I'm going to guess on a bullshit explanation like long ring island. It was something like "we got place to stretch so the animal are elongated". And Luffy having sun god abilities will translate to "don't you like to stretch in the sun?" I'm calling it
Yeah, hydras that are beheaded can regrow 2 new heads after all (in mythology at least), would be weird if his mythical df was just about being big and snakey
Even if BB didn't know of the alliance between yonkos, he still knows that BM went to Wano, as it was on the papers. So yeah it would make perfect sense for him to travel to WCI, at least to get his hands on the poneglyphes.
Heck man, I'm banking on your prediction! All the tension build up in this arc needs to get somewhere anyway, and that settles all the matters at hand with the BMP. Not sure about the sunny being destroyed tho.