Question: When do we receive the actual remission of sins?
Answer: Sins are remitted when we receive the Holy Ghost, not in the waters of baptism (McConkie, A New Witness for the Articles of Faith, p. 290.) It is the Holy Spirit of God that erases carnality and we become clean when we receive the fellowship and companionship of the Holy Ghost.
Which is why in D&C 109:21 it talks about repenting speedily, and returning to God as a blessing of worshipping in the temple, because it also says that we receive the fulness of the Holy Ghost as a blessing of the temple (v15). Both blessings still take place over a space of time obviously.