What is the significance of the corner stone
The corner stone is the foundational stone that unites two walls (cf. Mishna, Negaim 13, 2; 1 QS 8, 5-8).
This is the meaning of Isaiah 28, 16. Psalm 118, 22 has a different meaning: it's the finishing stone that crowns the constructions.
This metaphor of the last stone as sustaining the construction has found as corresponding (in arquitecture) to the construction of arches. If you take away the crowning stone the arch crumbles. So it’s possible that the last stone sustains the construction. You can see it especially in gothic Churches where these crowing stones frequently sport images of Christ and His Church.
Both verses are referred to Christ in the NT.
Psalm 118, 22 is used to prove the necessity of Christ's death and in order to express the reality of His exaltation (Resurrection and Ascension) (Mark 12, 10 par.; Acts 4, 11; cf. 1 Peter 2,7).
Is 2,20 is a prophetic text in order to express that salvation comes through Christ and the necessity of faith so we can be saved (1 Pe 2, 4ff; see Rom 9, 33; 10,11).
In Eph 2, 20 the Church appears as a construction whose foundations are the Apostles and Prophets. Christ is the crowning stone who sustains all the construction.