I have different take on the "win-win" conversation between lea and mouse in changes. I believe lea means win as in win harry over or harrys love, while mouse uses win as in winning the lotto or the best job. See it could be that mouses litter had a certain number of lets say slots to be filled and he feels that he got the best slot, job, or opportunity being he was so adventerous. Also it could be that mouse got the top job, guarding the outsider bane!!