As I heard, cup draw relates to qualities of your top 14 players, and league draw relates to qualities of all players. I'm not sure about CL but all opponents will be save league level as yours. There are plenty of discussions about this you may check more through this forum.

If you sell your now 5* player in next season, they will be available for lower league level teams, and will appear as 5* for L2 & 4* for L1.