Your problem is that you refuse to get into the heads of those who do not share your beliefs. As a result you do not understand them and you will be unable to change their minds.
That is incorrect. I teach all kinds of students. My atheists ponder what is being said all the time. And just last year two of them entered the Catholic faith. You are not asking the philosophical questions, instead you are asking theological questions and demanding sporadic philosophical proofs along the way. It is nearly impossible to meet your demands, when you demand theological answers and then gawk at how they weren’t philosophical enough for an atheist. What you seem to refuse to understand is that although philosophy and theology work in tandem, they still have separate approaches.
For my atheistic students I do pre-evangelization, which is philosophical. After a year, we are ready to do theological arguments.
Your real question seems to be why must there be a Church if God’s grace is sufficient. But this is not correct. God’s grace is necessary, but only God Himself would be sufficient. Until you have God Himself, you lack any fullness.
It is possible to have grace on earth, but not have God fully. If this is so, then grace before death is not completely sufficient. This is also clear because Adam and Eve were able to fall, even though they had sacramental grace.
At this point, accepting the Fall of Adam and Eve, we need to move to the theological idea of redemption, which is Christ. Why the Church? Really the question is why Christ? If you don’t understand the need for a material savior, you can’t understand the need for a Church. I have already given a lengthy response to why man needs a material savior, and spiritual prompting is not sufficient.
Obviously the former answer is a theological one. If you wish to ask a philosophical question, such as “why would we need God in the first place?” so that an atheist could wonder at the idea of grace, then ask it. Don’t throw a Red Herring Fallacy and create some fallacious vicious circle.
A philosophical idea of Church would be more like this:
What human being has no government, no form of order or management? If you have sports, you have rules to govern them. If you have a business, you have rules and a form of governance: must be to work by 8AM, or lunch break not to exceed half an hour.
So too even with spiritual concepts such as religion and mathematics and physics. There are governing laws and orders that give it shape and direction. Without which, there would be chaos and no purpose. It seems that religion, dealing with the highest purpose of man’s life, would also have a form of order and governance, seeing as all other aspects of spiritual concepts also has governance.
If human beings need directing, then just having a physical savior who lived once, long ago would not suffice. We would need continual direction. This is what Christ meant when he said to Peter: look after my sheep/flock. Without someone to direct and guide, it would be lost and no longer a flock at all.
I have to get back to teaching students, yes, atheists included. I fear I can spend little more time on this topic as I have already addressed your concerns and philosophically dismissed your ideas that sin come from God, that there is no grace without sin etc.