Okay, I think the comet scene actually was gem Phos from the past, like 74HC595 said. Ishikawa writes on the last page, "Phos will, as Phos does, continue to travel space and time, shining brightly all the while." And the first page says, "Far in the future, we became 'the lustrous,'" unless it's all just symbolic. Maybe reading too much into it.
Also, I like how in the page with the bees looking for Phos, they come up with their little arms crossed when they can't find him lol. Very cute