Sorry for responding piecemeal, it just saves me quoting your post in sections to identify which question each answer is specifically addressing, but I understand that your questions build to express a single point and will attempt to express a single point with my split responses.
Are Christians doing all they can to promote the Kingdom of God?
Probably not. Although it is probably worth pointing out the parable of the wheat and the tares at this point. Jesus said that there would be wheat (those destined to be gathered together and carried into God's storehouse) growing alongside tares, weeds that look like wheat when they are young, but very different when they mature (those destined to be gathered and destroyed). Thus when one looks at the Church, one sees the Visible Church, which is a 'garden' planted with a mixture of both 'wheat' and 'tares'. I do not offer this as an excuse, but just a caution that not everyone that calls them self a Christian really is one.
Of the 'wolves in sheep's clothing' or the 'goats among the sheep' or the 'tares among the wheat', without a doubt and with 100% certainty.
Among the 'true sheep' and the 'wheat' ... those whom Jesus calls 'his own' ... sometimes ... No point in putting lipstick on a pig and calling it a prom queen.
I don't know where this concept came from that Christians are 'better' than other people, or somehow more 'innately good' or even (please) 'righteous'.
Christians, I mean the real followers of Jesus ... as in 'my sheep hear my voice' or 'none may come to me unless the Father who sent me draws them' ... are 'HOLY'. An interesting word that means 'Set Apart'. That's it. We are set apart for God to use. He bought us, He claimed us, we are His property. We are NOT perfect. We are NOT innately good. We are NOT righteous (free from the tendency to do the wrong thing and need to ask forgiveness for it). So yes, from time to time, we are hypocrites.
If some poor sinner does get "saved" which church does he/she go to? With the Church so divided, how can anyone believe in any one of them as being the only one that is right? Does Jesus have "curses" for the lukewarm and the churches that have lost their "first" love?
Given my answer above and your question here, so what is the point? You have made the same mistake as the Jews that rejected Jesus. Their trust for their innate goodness and forgiveness was in the Temple and the Priests and the Law of Moses and the fact that they were descended from Abraham. You seem to be asking which Christian Denomination is the correct 'Tribe of Abraham' that will get you a free pass into ... whatever it is that you expect from God ... because of your membership in the 'club'. There is no correct denomination. There is no right Church. There is a person. God really became a man and he really walked the Earth and he really died for us. True Christianity is the reality that a dead man got up and walked and says that you can, too. His words are recorded in the Bible along with important advice from people who hung out with him and passed on what he taught them.
It really is all about Jesus.
1 John 5:9-12
9 We accept human testimony, but God’s testimony is greater because it is the testimony of God, which he has given about his Son. 10 Whoever believes in the Son of God accepts this testimony. Whoever does not believe God has made him out to be a liar, because they have not believed the testimony God has given about his Son. 11 And this is the testimony: God has given us eternal life, and this life is in his Son. 12 Whoever has the Son has life; whoever does not have the Son of God does not have life.
There you go, the secret handshake to get you into the club. It ain't about me or you being perfect (or even good). It is about trusting in the Son of God to fix what is broken and make us better. Thus the 'curse' for those who walk away from the Son. If you put a plastic bag over your head because you have decided that you don't want any more of the air that God has provided, whose fault is it that you suffocate? If you walk away from the only source of Eternal Life that God has provided, then whose fault is it that you have no eternal life?
Unfortunately, Christians, and their churches, can be the worst witnesses of trying to convince people that Christianity and their Bible is the truth.
Sad but true. Fortunately, it was never about the 'church people' but has always been about Jesus. It was never about the denomination, the Church is the people. The sheep whom the Father drew to the Son, who hear Jesus voice and listen, and are scattered across the Visible Church doing what we are sent to do. Each one, reach one, teach one.
If the Father chooses to draw you, you will not be able to resist. If the Father chooses not to draw you, you will never want to come. It is ultimately, not our job to convince anyone of anything. We are to give an answer for the hope that is within us and to make a disciple of anyone whom Jesus chooses and trusts to our care. [FYI: This falls under giving an answer]
