What does it mean that “no one is crowned unless he has struggled?”
It means that we must first be tested. We have to work hard to do what is right. Then we will be rewarded by God for our perseverance and obedience to His Will.
How were the angels tested?
We don’t know for sure.
We do know that the devil’s sin was pride.
We can guess that the fallen angels’ sin of pride was that they didn’t want to lower themselves to worship Jesus in His human nature, since humans are lower than angels.
Can fallen angels change their mind and return to God?
No. Since, they have perfect intelligence (smarter than all humans), there is nothing new for them to see that could change their minds.
They permanently choose to honor themselves above loving God, and for that they lose God’s joy and happiness for all eternity.