There's no contradiction in that text. You quoted 3:85, but didn't precede it with 3:84. 5:72 and 5:116 are mis-interpretations of 'Trinity Doctrine'. Christians aren't Tritheists and only worship the 'G-d' of Abraham.
5:72 and 5:116 don't refer simply to the trinity, since both passages include propositions that criticize the worship of Jesus and Mary, either as gods or as the son and mother of god, individually, not just as part of a trinity. These explicitly outline which believers in Jesus are going to hell and which aren't, thus explicitly contradicting your pluralist reading of 3:84 and 29:46. I would argue that said pluralist vision is your own eisegesis at work, even if you're drawing on historical context.
1. 5:72 refers to associates, and to God's identity with Jesus. Even without a trinity, if someone believes that God shares an identity with Jesus, that person is going to hell according to the verse. This verse clearly disputes the notion that God can be identified with Jesus.
2. In addition, as I'm sure you're aware, there are numerous verses in the Quran that reject the notion that God begat a son of any kind and claim that those who believe that he did are blaspheming.
From 1 and 2, it is clear that God's relationship to Jesus cannot be identified either as a father/son relationship or as the identity of the two, and to do either or both of those is explicitly labeled blasphemy. Therefore, any Christians who believe those things are blasphemers and incompatible with Islam. We also know that any teaching that includes those beliefs is not the teachings of the prophets. Thus, the identification of all the prophets and their teachings with one another in 3:84 is not an identification of all of the alleged followers of those prophets with one another.
As for Christians not being Tritheists, that's your assumption. What Christians believe about themselves may not square with the doctrines about the nature of God and the trinity. It isn't for either of us to say that Christians as a whole believe or don't believe anything.