You asked about the prepositions used in 1 John 5:20, and why NWT renders the verse the way it does. The verse reads in WH: οἴδαμεν δὲ ὅτι ὁ υἱὸς τοῦ θεοῦ ἥκει, καὶ δέδωκεν ἡμῖν διάνοιαν ἵνα γινώσκομεν τὸν ἀληθινόν· καί ἐσμεν ἐν τῷ ἀληθινῷ, ἐν τῷ υἱῷ αὐτοῦ Ἰησοῦ Χριστῷ. οὗτός ἐστιν ὁ ἀληθινὸς θεὸς καὶ ζωὴ αἰώνιος.
Others may havbe some points to add, and I might have more to say as well. But here are some initial comments that might help:
Benson Commentary: and we are in him that is true — In his favour, and in a state of union and fellowship with him; even — This particle is not in the Greek; in — Or rather; through; his Son Jesus Christ — Through whose mediation alone we can have access to, or intercourse with, the Father.
Barnes' Notes on the Bible: And we are in him that is true - That is, we are united to him; we belong to him; we are his friends. This idea is often expressed in the Scriptures by being "in him." It denotes a most intimate union, as if we were one with him - or were a part of him - as the branch is in the vine, John 15:4, John 15:6. The Greek construction is the same as that applied to "the wicked one," 1 John 5:19, (ἐν τῷ ἀληθινᾧ en tō alēthinō.)
Tyndale's NT (1 John 5:20-21):
We know that the son of God is come, and hath given us a mind to know him which is true: and we are in him that is true, through his son Iesu Christ. This same is very God, and eternal life. Babes keep yourselves from images. Amen.
Williams-New Testament in the Language of the People:
"And we know that the Son of God has come, and has given us insight to recognize the True One; and we are in union with the True One through His Son, Jesus Christ. He is the true God and eternal life."
See Thayer's Greek-English Lexicon here: http://biblehub.com/greek/1722.htm
Notice what entry 6b says.