As far as the second transformation, it seemed to be completely triggered by Orhime's cries for help. He became 100% fueled by his power and instinct at that point. To be honest, after seeing the anime filler, that transformation actually seems to make more sense now, although instead of his zankapto/inner hollow leaving him, he let their instincts take him over.
As far as being able to use that same hollow mask, it seems to be going along with his mask progression as we've seen before. I know it is in here, but after major fights, he has been getting additional red stripes on his mask. At the start of his fight with Ulq, he was able to open the mouth of his mask as well. This just seems to be another step (although a large one) in that progression.
Or at least, that is my best guess right now. Considering Kubo never explained the mask progression at all, I doubt this will be explained.