You start out with Eph. 3:10 suggesting this is “God’s purpose”. Reading the context we find:
Eph. 3: 7 I became a servant of this gospel by the gift of God’s grace given me through the working of his power. 8 Although I am less than the least of all the Lord’s people, this grace was given me: to preach to the Gentiles the boundless riches of Christ, 9 and to make plain to everyone the administration of this mystery, which for ages past was kept hidden in God, who created all things. 10 His intent was that now, through the church, the manifold wisdom of God should be made known to the rulers and authorities in the heavenly realms, 11 according to his eternal purpose that he accomplished in Christ Jesus our Lord. 12 In him and through faith in him we may approach God with freedom and confidence. 13 I ask you, therefore, not to be discouraged because of my sufferings for you, which are your glory.
First off: The fallen angels are not the rulers and authorities in the heavenly realm.
Why would God need to go through all this to “help” hell bound angels?
There would be powerful good beings in heaven wondering why Gentiles were treated so differently from Jews prior to the cross and so replacing the Jewish nation with the Church we have the unity desired, the mystery is made known.
The “intent of the Church”, does not equal God’s overall purpose.
You might want to read my post 23.
We are not being “Tested” but are here to obtain and use a purely charitable undeserved gift, which must be humbly accepted of our own free will as charity.
We have the privilege and honor of being part of God’s work (allowing the Spirit to work through us) to bring others to Christ, so they can make an informed choice. We are in the presence with God when He is glorified and share in His glory that way.