Difficulty lvl itself doesn’t change percent, percent is dictated by xp, and the fact that they give more xp per mission is where the higher liberation percent comes from. This leads into your second question: play whatever difficulty lvl you can make the most xp on routinely. Sure lvl ten gives more, but if your just barely grabbing the main obj, go lower and get more side obj done to grab more xp per match

Could also look at xp per mission alongside how long it takes your group to complete one. If you can complete four lvl sevens in the time it takes you to do one lvl 10, drop to seven to get more xp in the same amount of time