If this was all shot on GoPro then I want to learn how you do it. What did you use for lighting and why is there such strong vignetting along the edges and corners. Did you use a wet-lens for the close-up shots?
Indeed, makes no sense unless you need to get it perfect to within a few grams. Same for the rotation issue. Vacuum level can make a difference in theory. For instance if almost all the air inside is in a big dome port and little in the body, than increasing the vacuum will give more weight loss in the dome and thus causes the system to start tipping forward if it was perfectly balance without vacuum. I didn't look at the original post but it is hard to imagine a case where weight balance is so critical. Just moving a strobe a bit underwater will upset balance more.
There are a number of groupers with somewhat similar features. In the Red Sea, with which I am more familiar, it would be most similar to the greasy grouper (Epinephelus tauvina), but it isn't quite right. Yap may have a closely related species or E. tauvina may look a bit different in that corner of the Pacific. Hopefully someone familiar with that area can give a more definitive answer on what it is.