In regards to Nietzsche, so if you approach the question about the philosophy of Friedrich Nietzsche within the context of will to power, AND if one is working with a psychological interpretation of his theory, then the response is clear. No, because that would be zero sum thinking. From WP:
Zero-sum thinking perceives situations as zero-sum games, where one person's gain would be another's loss. The term is derived from game theory. However, unlike the game theory concept, zero-sum thinking refers to a psychological construct—a person's subjective interpretation of a situation. Zero-sum thinking is captured by the saying "your gain is my loss" (or conversely, "your loss is my gain").
In a metaphysical interpretation, things become less clear. You go on to ask:
It seems this supposes that our sense of power actively reacts and resists that of someone else, which strongly suggests it will be more active when their power is less. If it's at all reasonable, this reason to think morality is meaningless?
Indeed. Modern psychology have a parallel to the notion our sense of power actively resists, and that is related to the concepts of reactivity and reactance. From WP:
In psychology, reactance is an unpleasant motivational reaction to offers, persons, rules, or regulations that threaten or eliminate specific behavioral freedoms. Reactance occurs when an individual feels that an agent is attempting to limit one's choice of response and/or range of alternatives.
Therefore, from a highly naturalized epistemological approach, one can argue that psychological altruism even from a Darwinian perspective suggests that morality is necessary and advantageous, and that the conflict inherent between an individual's needs and society's needs are in tension necessarily. This of course, moves the conflict of will to power to groups of people, and brings us neatly to another bias related to zero sum bias, the bias of in- and out-group thinking. Now, we see why morality is not meaningless, because it derives its meaning from the social-psychological purpose of morality, an organizing force among individuals.
Thus, morality, which we are biologically wired to have, is a sociobiological phenomenon, and is not meaningless, but meaningful and impactful. This sheds light on the struggle of organized religion, and ties in Nietzsche's objection to religion as an opiate of the masses neatly. Freedom, free will, and will to power is endemic to political philosophy, and it should be no surprise that followed to its ends, liberalism leads to critical theory in its modern practice.