Well I'm sure the difficulty affects everyone differently depending on what they have/don't have, but it's pretty hard to see someone clearing 10 and just not being able to clear 11 unless it's because they did 10 with their good team, and then start the cycle again trying to clear 11 with a subpar one. The mechanics of 11 isn't all that difficult, just don't burst the first mob, and bring some healing for the longer fight due to beefier bosses.

As for the second question, the missions count on clearing, so if you weaken with an off-element team, and then bring the right team to complete the missions to clean up, you get all the rewards.