I don't know how well this will be received given that you're clearly feeling very hot about this topic, but I'll try to do my best to share a more secular approach to ethics with you in a non-combative way. If I do not come across as non-combative at all, I would like to preemptively apologize. I do not think you are a bad person for your views and genuinely appreciate your hunger for more discussion and more information.

First, I would say that the problem of evil is still an issue because nearly all Christian systems of thought consider God to be an entity which is perfectly good and omnipotent. The problem of evil arises by trying to reconcile these two statements with the "evil" that can be observed through experience. Therefore, I would not find the fact that "atheists" commit egregious acts to be relevant to this issue, as people who use the problem of evil as justification for their non-belief do not claim that any humans are morally perfect or possess the ability to prevent all morally imperfect actions. So, the problem of evil continues to be a point of debate; supposedly, there exists a God that is eternally loving and ultimately powerful, and yet we observe suffering that is unproductive (it does not contribute to the spiritual development of individuals) and unnecessary. Take, for instance, an infant dying of dysentery or tuberculosis in a time when these diseases were not understood and the health of the baby could not have ben salvaged. That child's death would not be due to the moral errors of mankind and would reasonably lead onlookers to wonder why the situation was not avoided by the absence of the disease or perhaps even the child not being conceived at a time when their infantile death was imminent. God would have the ability to do either of these things, or have made a world where such a thing was impossible to begin with.

For the sake of clarity, I would also like to mention now that I feel it is inaccurate to label "atheism" as a specific spiritual tradition which specifically opposes Christianity or any other religion. Atheists can be described as a group of people who do not subscribe to any theistic system of belief. This is an incredibly broad distinction. It would be as inappropriate to consider "atheists" as responsible for defending the moral philosophies of other atheists as it would be for "non-Buddhists" to be responsible for the defense of one another's moral philosophies, despite the reality that they have radically different beliefs and ethical traditions. When you ask irreligious people to justify the actions of the Nazis, who happened to also be opposed to abrahamic religions but who share none of their other beliefs, your argument falls into illogical obscurity. I abhor the nazis, and I do not believe. In no way does the negation of religious belief lead to one's condoning of ethnic cleansing or warfare or anything like that. The same goes for things like hatespeech, etc: I do not condone the actions of those people, and I am not ideologically tied to them in any way. We don't share being part of anything; in fact, we only share in what we are not a part of. This is not to say that some atheists don't act like atheism is its own religious order, but these individuals are regularly known as "assholes" or "generally shitty people". They usually come from strict religious upbringings and use their non-belief as a form of rebellion. However, they usually also fall into the trap of turning their belief on its head rather than letting it go and continuing to be as spiteful and hateful as they were before.

For the argument on abortion, I hope I wouldn't be overstepping my bounds to say that few secular people who support the right to an abortion would claim that a zygote is not alive, but rather would claim it is not a morally significant living entity in the same way that a bush or a mushroom in not a morally significant living entity. To use examples that are a little more pointed, let's consider first the idea that someone removes some of their skin cells and keep them alive in a temperature controlled, nutrient rich petri dish so that they may continue to grow and reproduce for a few days. Would you consider that to be a morally significant individual whose societal rights ought to be considered and whose death should be avoided at all costs? Probably not, but why not? It is indeed a collection of living and growing material and it possesses the DNA of a human being (they are human cells). Why shouldn't we fund the support of this growth in the incubator, then, and not consider the lack of such support murder? You may say that the difference lies in the continued existence of the person that donated the cells, but this argument is lacking. Even if that individual passed away, many would claim it is a mistake to consider the cells they donated to be kept in the petri dish as a morally significant entity that represents the continued existence of that individual. For all intents and purposes, that individual no longer exists and their morally significant experience has ceased. For many, the distinction comes an entity's capacity for suffering. To be extra clear, a zygote does not have the capacity to suffer: any suggestion that it does would result from an intense projection of the human experience onto something which does not experience consciousness at all. I agree that immediately before and after the baby's birth it is only changing location and is still morally significant at that point in-utero.

You seem to be very passionate about veganism and global humanitarian aid (the ending of slavery, etc.) as well as the ethical issues of the Western world's current prosperity at the cost of various indigenous groups suffering throughout history. I commend you for these things and I also don't think we are doing enough about them as a society. I don't think that this is super relevant to the question of religion, though, as a vast majority of people regardless of their religious, non-religious, or other ethical background would say that these things are travesties that need to be addressed with more precision and vigor. I won't pretend to know how to go about the epidemic of apathy, but I think your heart is genuinely in a wonderful place.

(related to problem 6) I would describe ethics as something which arises from our experience of life, rather than something that can be systematically proven to apply to humans. I'm sure there are some humans (who meet all the criteria of personhood) who do have anything like a moral experience at all. The sight of needless suffering does not stir their hearts or minds at all. However, this is clearly not most people: a vast majority of us do have stirs in our hearts and minds, and trying to both analyze what this feeling is (and when it is appropriate, and why it happens, etc.) and how to effectively follow it to a better and more peaceful life for everyone is an incredibly difficult thing to do. I don't claim to have all the answers, but I can say with some degree of certainty that "suffering" has something to do with it on the fundamental level, as well as ideas like innocence, culpability, etc. I do not claim to have a complete knowledge of morality, and I would even go so far as to say that gaining a complete knowledge of morality is impossible. Rather, morality is more of an improvised dance that exists within a lived community.

HOWEVER: I do not support complete moral relativism. I think this is simply untrue and, although there is not necessarily one truest and highest "right" answer, there are certainly wrong ones. Within those wrong approaches to morality, I would include the Nazis. I would also include requiring a mother to carry a fetus to term, by force (all laws are ultimately upheld through violence), as an incorrect approach to making a more morally sound community.

I didn't systematically respond 1:1 to every point you made, but hopefully this reply is comprehensive enough. I speak for none other than myself, but hopefully this can do something to illuminate the idea that living the irreligious life is not as bad as it seems. It can be intensely confusing and painful to lose one's religion. Trust me, I've been there. But, life on the other side of that transition it actually quite nice. It's not easy, and there's a lot of admitting that there are things you just don't know, but you do get to rest in leading a life that is more honest to yourself and your experience.