JWs interpret the scriptures slightly differently than others, I don't think I would say that they are not Christians, because there are many different kinds of Christians each with their own interpretations.
Being an atheist, it personally doesn't matter to me who believe they are correct, because I think all of them is wrong, so that at least speak to my neutrality in regards to this topic
Personally, from reading the bible there are things which I agree with that JWs have gotten right. I do not think that the bible support a trinity and that JWs are correct that Jesus is the son of God. Jesus himself doesn't seem to think anything else and also the scriptures are written in a very strange way, if he were in fact God.
Whether Michael is suppose to be Jesus or not, I have no clue given that Revelation is such a weird text that I think almost anything goes in it.
The reason they reject the cross or any symbols as far as I know, is because God made it very clear that you are not to worship any idols, symbols etc. Also why you don't find Jesus on the cross or other things like that in JWs "churches".
Exodus 20:4-6
4 - “You shall not make for yourself a carved image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth.
5 - You shall not bow down to them or serve them, for I the LORD your God am a jealous God, visiting the iniquity of the fathers on the children to the third and the fourth generation of those who hate me,
6 - but showing steadfast love to thousands of those who love me and keep my commandments.
So if Jesus is God, then clearly all other Christians are breaking God's command by having crosses and Jesus in the churches etc. So again, depending on how you interpret it, JWs are not wrong here, because the Bible does actually say that.
Hell as far as I know, comes from the word Gehenna. Which was a place outside Jerusalem where they burned stuff.
The word derives from the Hebrew: גי(א)-הינום Gêhinnôm (also Guy ben-Hinnom (גיא בן הינום) meaning the Valley of Hinnom's son. The valley forms the southwest border of ancient Jerusalem that stretches from the foot of Mt. Zion to the Kidron Valley. It is first mentioned in Joshua 15:8. Originally it referred to a garbage dump in a deep narrow valley right outside the walls of Jerusalem where fires were kept burning to consume the refuse and keep down the stench. It is also the location where bodies of executed criminals, or individuals denied a proper burial, would be dumped. In addition, this valley was frequently not controlled by the Jewish authority within the city walls; it is traditionally held that this valley was used as a place of religious child-sacrifice to Moloch by the Canaanites outside the city (comp. Jer. 2: 23).
Like Sheol, Gehenna is sometimes translated as Hell.
Whether that applies to all cases in the Bible I don't know, but it is also not impossible to imagine that the meaning changed over time, from what it originally referred to. And as far as I know, JWs do not believe in Hell for this reason, because they believe it refer to this place outside Jerusalem. Again, hell in the OT as we know it from the NT does not exist. So again, are they wrong or right, depends on the interpretation.
This argument you could use against any Christians, why did God wait to reveal himself to the Jews and not those before him?