Let’s clarify our terms here. Epiphenomenalism is the belief that mental experiences of choice and freely willed action are basically illusions. They are the accidental side effects of purely physical events and cannot be the source of any action or event. In other words, I may believe that I am choosing to type these words now appearing on the screen, but in fact, the words are being created by a complex set of physical/chemical reactions –I merely think that I have the freedom to type whatever I wish.
It would be a mistake to call this view scientific in the first place; since it rejects, without adequate explanation, a large pool of data –that being your own subjective experience of mental agency. It may be true that you cannot prove that the people you see around you are not cleverly designed automata that merely appear to have internal mental states. I feel safe in assuming, however, that you do experience yourself as possessing both consciousness and will. Before rejecting that experience, you would need a theory that adequately explains why certain configurations of atoms (people) appear to behave as though they have mental states, and furthermore why you personally experience some phenomena, such as the movement of your arms and legs as under your control, and others, such as the weather or the movements of other human beings, as impervious to your will (in other words: if your sense of agency is an illusion, and you are merely an observer of all things, then why does that feeling of control or perception of agency extend only to some of the many things you observe?).
Epiphenomenalism provides no such explanation, rather, it amounts to a blanket statement of belief (or rather, disbelief) on the subject of mental agency. In fact, the situation is even worse, since if epiphenomenalism were true, it could not be proved in any meaningful sense, give that the demonstration of proof would be (in that case) as empty as any other mental illusion.
Given this, one practical solution to your dilemma would be to adopt a version of Pascal’s wager, as follows: If, on the one hand, epiphenomenalism is true, then your beliefs and actions will be determined by events beyond your control. In that situation, your decision to believe in epiphenomenalism and behave accordingly will be equally as meaningless and predetermined as your decision to reject epiphenomenalism. On the other hand, if epiphenomenalism is untrue, or simply flawed in ways we do not currently understand, then your decisions and actions do have validity, and potentially make a significant difference in the world. Therefore, you are justified in acting and behaving as though epiphenomenalism is false –if you can! –no matter what the true state of affairs.
Although epiphenomenalism is a relatively new philosophical position, it raises issues similar to those faced by anyone who believes that the universe is deterministic in one way or another, whether that be because the physical trajectories of the subatomic particles are held to be predictable and unalterable, or because the story of life is held to have been prewritten in indelible ink in God’s diary. In each scenario, however, my answer would be the same. Whether or not we live in a way that is fated, the only option that makes sense is to live as though our freedoms of choice were momentous and absolute.
My sense, however, is that your real question is how to reconcile your belief that we live in a fundamentally physical universe with your sense that mental or metaphysical entities such as values and ideals do make a difference, I would suggest that you look into “Emergentism,” the idea that irreducibly complex behaviors can arise from simple foundations –as in the mathematical phenomenon of fractal geometry. From that standpoint, it is reasonable to hypothesize that a mental phenomenon such as consciousness might emerge from a purely physical ground, yet not be reducible to something understandable in a purely physical framework; and even that a higher-order pattern (consciousness) could influence a lower-order pattern (physical matter) through the kinds of complex feedback loops that are ubiquitous in chaotic and emergent systems (rather than through “mysterious energy”
or whatever other non-scientific channel of causation you fear embracing mental states would commit you to).
Furthermore, if we take ourselves to be patterns of emergent complexity, there is in fact an underlying foundation for morality that attaches to that conception, to the effect that some choices –the ones that allow us to live in harmony with other complex patterns –are positive in that they increase the amount of complex order in the world, while others –the ones that destroy –are negative.