Answer

There are positive commandments that are procedural commandments. For example, positive commandments 95 (annulment of vows) and 96 onward (the laws of impurity). Maimonides himself writes there that this is neither a command nor a prohibition, but only a definition, and yet he still classifies it as a positive commandment. It is clear that a man who annulled his wife's vow does not receive reward for that, because he did not do something positive. The commandment merely defines what someone who wants to annul must do. If he does not do it, then he simply did not annul it, and that's all.
As for the commandment of divorcing a woman by a bill of divorce, it is commonly assumed that this is a procedural commandment. If so, then what I wrote above applies here as well. But in Sefer HaChinukh, at the end of the commandment of divorce, he wrote that there is a punishment for one who divorces not in this way, and from his words it is clear that this is an actual commandment. In an article I once explained that he means to say that one who divorces his wife—that is, sends her out of his house (divorce in the everyday sense, not the halakhic one)—must also permit her to the marketplace, meaning give her a valid bill of divorce according to Jewish law. And if he does not do so, he is punished. I am not sure whether, when he does do so, he receives reward. There are positive commandments that are a prohibition inferred from a positive commandment, meaning a commandment such that if you neglect it you are punished, but if you fulfill it you receive no reward. For example, the commandment to eat Sabbatical-year produce, which the Sages expound from the verse, "for eating"—and not for commerce. Meaning: one who eats does not thereby fulfill a commandment, but one who trades in it has neglected the positive commandment of "for eating."