PS: If you throw your name out there for friend requests, after about 20 or so the server info retrieval takes a really long time to process, which makes getting to the Challenges page really difficult. Might want to de-sync updates by letting players pick their own refresh rate, or keep rewarding chests.
Rewards for challenges should all be awarded if you've passed each stage. So if I beat the 5th wave, I should get all of the rewards for the previous 4 as well (since the challenge closes and I can't play it again)