I think the reward comes in the form of coin. Try clicking your coin and you should see it

QR codes from e wallets will automatically refresh every 60 seconds, and if I'm not wrong, the code will become unusable once you quit from the screen where it shows the QR code.

I believe you won't be using it either if they allow people to screenshot their QR codes for payments, that means anyone can just steal your QR code and use it.

I once saw 2 aunties blocking the whole queue to pay and redeem something with TnG in 7-11 using someone's code in a screenshot that someone sent to them, cashier kept saying error but those bastards kept insisting on trying it using few screenshots and they wondered why it still couldn't go through, too bad wasn't in the mood to rage at them that time.

anyway, we can't undo the shake and retry as no one can proof their statement is correct or wrong, just believe what is the best for u to shake it one by one or accumulate few to shake
some mission got limit per acc, once u finish it won't appear in your mission list

Blue colour mission is different