Right, it's hard to tell because the raws itself are in Korean. But as of now (as far as i read the korean ver), the MC (the guy who enters the fantasy world) seems to be going back in time (fantasy world) in a male form, where William was still a kid. Although I could be mistaken because I'm not sure if that kid is actually William. Meanwhile, the modern world continues to make cameo appearances as usual.
As for the ending itself, I still can't picture what will happen—whether they will come back to their respective worlds or never come back and stay. But for now, I think it's more likely that the first option will happen. But I don't know for sure, it just more make sense for the 1st option to happen.
I can't quite picturing what will happen at the end, this is new, because usually when the MC from modern world enter the fantasy world, the other soul was vanished/gone. But this one, both are present and literally switch soul. Plus, they have their own story for their journey.