Of course they can. It won't help but they can. I can call myself Mr President, buy that will not enable me to veto legislation.
No it is absolute and imperative.
No it doesn't. Becoming a Christian is identical in Catholicism and Protestantism. It is afterwords that differences appear. Those two make up 90% of Christianity. Most differences involve more secondary issues like music in Church or transubstantiation.
Since Jesus said all that are born again by grace through faith belong to him and the church then that is the criteria. Our secondary differences are incidental concerning becoming a Christian. I reject the tone of these statements. They seem to imply that since some differences exist let's all give it up as meaningless. If that is the case the practise of law, medicine, physics, mathematics, politics etc... must be given up as well. I was spiritually born again (saved) by faith because I did not listen to statements like this.
I disagree with all your points.
"Since Jesus said all that are born again by grace through faith belong to him and the church...." I Doubt Jesus ever used those words, as no church existed during his life on earth.
I do not share your limited view of Christianity.
Like it was for Jesus himself, the centre of worship for a Christian is God.
Jesus as a Son of God, taught us how to serve God through our lives.
It is this service guided by the Holy Spirit, not a particular set of beliefs that is important.
I do not hold with any "Calvinistic" beliefs. These change Christianity from an active to a passive belief system.
We are not saved in this life, born again or otherwise.
I do not confuse a certain potential with a certainty.