See your errors after practicing
I think it'd be good to be able to see all the mistakes you've made after you've finished your exercise, especially with timed practice, since you don't really have the time to look and understand what you did wrong.
I really love the proposal! Maybe it is even possible to give a short summery on a higher level. E.g. pay attention that a certain noun has a different gender, or don't forget gramatical construct xx. In a later duolingo version, this could be extended across different exercises and give further feedback on personal weak points.
The whole point of timed practice is to force your brain to think quicker, so that when you will need to use the language in real life, you will be able to translate and compose sentences faster, because the enviroment is far less stressing. If they would allow you to stop the timer in order to see the mistakes you've made, that sense of stress would be erased, and your brain would relax. And, in that particular training, in which the goal is to make you think faster, is bad. If you want to check your mistakes, you can easily choose to practice without a timer.