Why would Paul approve of proxy baptism when he made it clear baptism does not save in the first place?
I don’t think he said this in chapter 15.
He said it much earlier in this epistle: 1 Cor.1:
14 I thank God that I baptized none of you, but Crispus and Gaius;
15 Lest any should say that I had baptized in mine own name.
16 And I baptized also the household of Stephanas: besides, I know not whether I baptized any other.
17 For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.
Paul makes it clear that baptism has no part in salvation. If salvation was brought about through baptism, why would Paul say he thanked God that he didn’t baptize any of them? Baptism either by a living person or for the dead has no part in salvation.
No where in scripture does water baptism indicate salvation. Again, Philip dictates the requirement for baptism is salvation before baptism:
Acts 8::
36 And as they went on their way, they came unto a certain water: and the eunuch said, See, here is water; what doth hinder me to be baptized?
37 And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God.
38 And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him.
This doctrine cannot be any clearer.