Up until now I could follow the story quite well, but now I'm lost. They went further back in time, when the island didn't sink. Maybe even before they arrived for the first time from their own perspective. And this Imase is his time syncronated Imase (Imase#2), with whom he did his last time jump, as they both confessed. So, we are going for another loop, with both of them hiding from themselves, with the exception of older Yamada (Yamada#old), as he knows they are there? So maybe this Yamada#2 was that Yamada that appeared to Yamada#1 with that mask and mocked him.
He also gets both girls. He bangs Imase and after there relationship gets sour, he just fetches girlfriend #2 (aka best girl) freshly from the refrigerator. Aren't timeloops nifty? Best thing even though he is mid 20 he gets a 16y girlfriend to screw with. Somebody call the cops.