I will address your argument.
Thank you for actually taking a stab at it.
First your argument assumes that the purpose of a material existence is likely to be hedonistic.
Nope. I have never made any claims that materialist people are likely to be hedonists. In fact, I said the opposite, most materialists are not like the materialist condemned in the Gita. Rather I said, citing Dawkins in support, it is only when one truly lives by a materialist worldview that it entails hedonism.
You have then backtracked and qualified this by saying that the materialist worldview allows for such.
No, I am saying this is a logical implication of a materialist worldview. The rest is just rhetoric. If who you are is the body, then your bodies needs are your needs. The body has no need for "truth, love, wisdom, beauty, art etc" its needs are survival, like among all animals. Which animal preens itself in front of the mirror? None, because it is not a need of the body.
Unfortunately, this tells us nothing. All views allow for such. You suggest there are consequences in other religions, but nothing prevents one from living a hedonistic lifestyle consequences be damned. In other words, a person can still choose to stick their hand in the lava with Hinduism so their worldview allows for the murder of the old lady as well. So the mere allowance of such actions is not an argument against a particular worldview in this case.
The excruciating pain one experiences by putting one hands in lava is the
deterrence. It is universal across all life, that no life likes pain, it avoids pain and seeks pleasure. It is true for an ant as it for a human. Sure, there are some stupid humans that give themselves pain, but only within a certain threshold. If I got a hot knife and digged it deep in you and twisted it inside you, you are not going to say "Oh, yes, give me more baby" you will scream in agony and try to get away. It is a natural instinct of the body. The mind can say anything it wants, even rationalise pain, like what you just said something silly that people would choose to stick their hands in lava, but the body will react immediately as soon as the skin even touches the lava.
Likewise, because we avoid pain, we would not knowingly put ourselves in a situation that will give us pain. In Hindu ethics, I will face exactly all the suffering I caused the old lady and her loved ones myself, not to mention the in between live stays in hell where I will be punished. I might come as an animal and spend another few billion years living as lower animals in the harsh world of the animal kingdom, constantly getting killed and eaten. ---- Why would I want I risk inflicting that on myself? Of course I don't want that. Therefore, I will not rob the lady and kill her.
Next that there is no consequence for actions is unrealistic. Their are always consequences for every action. Assuming a person can "get away with something" is vague at best. But, this is simply not the case.
What are you talking about, criminals get away with crimes all the time. This is an extremely unfair world where atrocious and unfair things happen e.g. the near total genocide of native Americans in America, which is now populated by Europeans. The people that tortured and brutally killed the Native Americans are now the owners of America and Native Americans second class citizens. Extremely unfair and atrocious, but that's reality.
Next, collaboration is beneficial. While there will be those who are free riders and seek to exploit society, in general it is our best interests to collaborate. Therefore, even assuming a completely hedonistic purpose one can better pursue such an interest if they play by the rules if you will so while occurrences of exploitation might occur locally, the general movement is not in that direction whether we are talking about atheists or Hindus.
Ok, I get this point, but even collaboration only insofar as it is beneficial to you. If there are times where I can break the laws where it is beneficial to me, then I should using the same ethic. In this case, it is assumed in the example that nobody is going to find out you killed the old lady and robbed her jewellery. You are a tourist in a foreign land and she lives in the middle of nowhere by herself. Nobody even knows you stayed over at hers. Therefore, this is near 100% risk free. You will come out millions richer. So why not do it?
Lastly, you overlook that the human is a complex creature. We have attachments and emotions that are pervasive in every aspect of our being. These emotions are stronger than simple cognitive thought processes. This is why people in prison, regardless of religious affiliation, acknowledge that they thought very little of the consequences when they chose to act. They knew they could be imprisoned, they knew they could even for fit their lives, yet they chose to act regardless. Adding a worldview with next life or after life consequences will not change this as it is a cognitive thought process that you are anticipating will halt an emotional one. When this does happen or even when it fails to happen, it causes anguish and guilt.
Lets approach the dilemma in another way. Suppose I am one of those materialists who says "Yeah, we only live once, everybody is going to die anyway, the old bat will die soon as well, and nobody is ever going to find out, and I will be millions richer and can life a comfortable high life from now on, so I am going to rob her, kill her and bury her in the backyard"
Can you persuade me not do?