Here's the complete quote surrounding amalgamation.
But if there was one sin above another which called for the destruction of the race by the flood, it was the base crime of amalgamation of man and beast which defaced the image of God, and caused confusion everywhere. God purposed to destroy by a flood that powerful, long-lived race that had corrupted their ways before him. He would not suffer them to live out the days of their natural life, which would be hundreds of years. —Spiritual Gifts, vol. 3, page 64
If you look at Ellen White's other uses of the same word, amalgamation, they were specifically in reference to the various means of corruption of by Satan.
Those who profess to be followers of Christ, should be living agencies, co-operating with heavenly intelligences; but by union with the world, the character of God’s people becomes tarnished, and through amalgamation with the corrupt, the fine gold becomes dim. When worldly agencies are introduced into the church, it is evident that Satan is carrying out his devices, working through those who profess to be followers of Christ, making them ready at any time to engage with him in disheartening and discouraging those who are faithful, who would stand wholly on the Lord’s side. —Review and Herald, August 23, 1892
Genesis 3:18—Amalgamation Brought Noxious Plants—Not one noxious plant was placed in the Lord’s great garden, but after Adam and Eve sinned, poisonous herbs sprang up. In the parable of the sower the question was asked the Master, “Didst not thou sow good seed in thy field? how then hath it tares?” The Master answered, “An enemy hath done this.” All tares are sown by the evil one. Every noxious herb is of his sowing, and by his ingenious methods of amalgamation he has corrupted the earth with tares. —Bible Commentaries, vol. 1, page 1086.2
I think what Ellen meant (the source material) is clear enough in her writings.
As to why James adamantly supported the work of Uriah, the 1800's was a different time and people thought differently than we do now.
Since none of them are here to clear this up, that's all I have for you.