I don't think any European power could exert force in the Far East even in the tens of thousands strong until well into the 1600's, if not later. By that point, domestic Japanese gun production rivaled, if not exceeded, that of any European power of the time. Honestly, the more I read about pre-Meiji Japan, the more I get the impression that if Japan ever fell behind the West in any significant way, it wasn't until the early 1800's, and didn't last all that long. And of course, Korean collaboration with Japanese occupiers only came after decades of Japanese intrigue that the Korean government couldn't very well stamp out because of the threat of Japanese force. The Japanese would have had a much easier time rooting out European-supported subversion in their own country if it came to that.