Matthew 28:19 appears to be very questionable when examined alongside other Scriptures and a 2nd-3rd century writer's words.
Several sites such as this one, http://www.tektonics.org/lp/matt2819.html give a good case to support the claim that the Scripture (as presented) is a fiction, just as 1 John 5:7 is.
NOt sure what you mean here since that site states:
The Epistle of Ignatius to the Philippians, in Chapter 2 (see here) says, Wherefore also the Lord, when He sent forth the apostles to make disciples of all nations, commanded them to "baptize in the name of the Father, and of the Son, and of the Holy Ghost....".
Tertullian, c. 200 AD (see here writes in On Baptism, Chapter XIII: "For the law of baptizing has been imposed, and the formula prescribed: "Go," He saith, "teach the nations, baptizing them into the name of the Father, and of the Son, and of the Holy Spirit." and in Against Praxeas, chapter 2 says, "After His resurrection ..He commands them to baptize into the Father and the Son and the Holy Ghost".
Hippolytus (170-236 AD says in Fragments: Part II.-Dogmatical and Historical.--Against the Heresy of One Noetus, "gave this charge to the disciples after He rose from the dead: Go ye and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost."
Cyprian (200-258AD) in The Seventh Council of Carthage Under Cyprian says, And again, after His resurrection, sending His apostles, He gave them charge, saying, "All power is given unto me, in heaven and in earth. Go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost." and alludes to the same passage in other places as well.
Gregory Thaumaturgus (205-265 AD) in A Sectional Confession of Faith, XIII (see here says, "....the Lord sends forth His disciples to baptize in the name of the Father, and of the Son, and of the Holy Spirit?"