15:24-28 How are we to understand this?

The end here refers to the end of Christ's millennial reign when all enemies of God have been defeated and cast into the lake of fire at the great white throne judgement (CP Rev 20:7-15).

At this time Jesus will hand the kingdom over to God. Heaven and earth will be renewed and together, Jesus and God will reign over the kingdom in the new earth for all eternity (CP Dan 7:13-14; Rev 21:1-3, 22-23; 22:3).

We should note in closing here that the one excepted from being put under Jesus' feet is God Himself (see also comments on Rev 20:4-6, 20:11, 20:11-15, 21:3-8).

15:29 What is the meaning and purpose of baptism for the dead and is Paul validating the practice here?

Paul offers no opinion on whether being baptized for the dead is right or wrong here and there is no elaboration on the practice anywhere else in scripture so we cannot speculate on its meaning and purpose.

Paul is simply showing up the inconsistency of the Corinthians; they rejected the doctrine of the resurrection of the dead, yet embraced the practice of being baptized for the dead. It was a contradiction in terms - if there is no resurrection, why be concerned for the dead at all (CP V12 and V20 with V29).

Also, why would Paul risk death for the gospel's sake if there is no resurrection of the dead and this life was all there was. If there is no life to come Christians would be better off eating, drinking and being merry before they died (CP V19,30-32 with Jn 3:3-5; Ac 26:23; Ro 4:24-25; Eph 1:19-23; 1 Pe 1:3-5, 21).