But HOW could restoration, rehabilitation or (especially) salvation of the wicked even be possible?

It couldn't! Thus the need for Hell.

Hell is the threshing floor of the wheat harvest and the winepress for the treading of the grape harvest in the lake of purifying fire.

No sinner will leave that lake of fire. But that fire will consume all sin and cleanse each sinner until they are clean. (see Purpose of Yahweh's Fire). All sin and every sinner will be destroyed, leaving only what is pure. As each is cleansed, the pure will leave the lake, reborn by fire, and enter the Holy City whose gates are never closed - open wide in invitation - welcoming the prodigals back to their Father (see "The Lost (Prodigal) Son").

Therefore thy gates shall be open continually; they shall not be shut day nor night; that men may bring unto thee the forces of the Gentiles, and that their kings may be brought. ... The sons also of them that afflicted thee shall come bending unto thee; and all they that despised thee shall bow themselves down at the soles of thy feet; and they shall call thee, The city of the Lord, The Zion of the Holy One of Israel. Isaiah 60:11-14

And the gates of it (that great city, the holy Jerusalem) shall not be shut at all by day: for there shall be no night there. Revelation 21:25

There is only one reason for the gates to be left open and we are specifically told that they are left open.