Hi, thanks for playing and feedback! Actually that was "last minute update" issue and we already fix it. Now you can return to previous missions for farm or fan or both:)
You should not incentivise players to farm levels they have already beaten. In fact I wouldn't even allow it. Instead you should make sure the first part of the next level is doable and rewarding for players who did the entire previous level. Rewards don't seem to go up at all with missions, which is odd.