The interpretation one arrives at is simply that the Lord is a constant comfort to those in need at any time, always. A suggested analogy with the English idiomatic 'for ever and ever' cannot be entertained, since in that phrase we have repeated the same word 'ever', while in the Hebrew phrase being examined 'olam' and 'ad' are two different terms. Even if it could be proved that the phrase 'olam wa ad' formed an idiomatic expression meaning infinite duration, that would still not show that 'olam' itself meant eternal. A more likely explanation seems to be that the author wished to express his concept of the inexhaustible unlimited power of Yahweh and knowing that 'olam' did not signify infinity, added 'wa ad' 'and beyond', or 'further still', so that his readers might de directed to think of duration beyond that usually covered by 'olam'. The dozen or so litugic repetitions of 'His mercy endureth le olam'.
Mercy relates to sinful humans.
No one supposes such folk to have existed eternally. God's mercy operates in human need. The liturgy would recognize its availability at all times - perhaps conditioned by repentance and faith. Some writers have claimed that because of the development of new concepts regarding the 'eternity of God', 'olam' is here used to denote temporal infinity. A careful systematic objective study of the usage of this word and other expressions in theological contexts in Deutero-Isaiah tends rather to a different view.