Regarding women’s jewelry on the Sabbath, I meant this law:

A woman who goes out with a perforated needle is liable, while a man is exempt. And a man who goes out with a needle that is not perforated is liable, while a woman is exempt—because it is one of her ornaments, and it is prohibited only by rabbinic decree, lest she show it to her friends. This is the principle: anyone who goes out with something that is not his ornament, and is not worn as clothing, and carries it out in the usual way such an item is carried—is liable; and anyone who goes out with something that is one of his ornaments, if it is loose and might quickly fall and he may come to carry it in the public domain, and likewise a woman who goes out with ornaments that she commonly removes and shows—these are exempt. But anything that is an ornament, does not fall off, and is not ordinarily shown off—it is permitted to go out with it. Therefore, one may go out on the Sabbath with an anklet or bracelet worn on the arm or leg, provided it is snug against the flesh and does not slip off. And so too with anything similar.

And the Shulchan Arukh wrote: Shulchan Arukh, Orach Chayim, Laws of the Sabbath, section 303, paragraph 18:
Nowadays, our women have the custom of going out wearing all kinds of ornaments. Some say that strictly speaking they are forbidden, but since they will not listen, it is better that they remain inadvertent sinners rather than intentional ones. And some have defended them by saying that they act in accordance with the last view I cited, namely that the prohibition against going out with ornaments applied only to a courtyard that is not enclosed by an eruv; and now that we do not have a true public domain, all of our public domains have the status of a karmelit and the law is like that of a courtyard without an eruv, so it is permitted. Gloss: And some give yet another reason to permit this: nowadays ornaments are common and women wear them even on weekdays, so there is no concern that perhaps she will remove it and show it, as in earlier times when they were not accustomed to go out wearing them except on the Sabbath and they were not common (Tosafot, chapter "With What May a Woman Go Out?" and the glosses to Alfasi there). However, a ring with a seal for a woman, and one without a seal for a man—about which the Mishnah says that one is liable to bring a sin-offering—even in a karmelit it is forbidden, even for us. And the same applies to anything else regarding which it was said that one incurs liability for a sin-offering. And some say that nowadays, since men have the custom of going out wearing a ring without a seal, it is considered an ornament for them, and is permitted. According to this, it is possible that since nowadays women have the custom of going out with a ring that has a seal, it too is considered an ornament for them, and is permitted. Nevertheless, women must be warned not to go out except with pins that they need in order to hold their garments together and no more, because in matters from which they derive no benefit they will listen to us.

Regarding the permission to sell livestock to a non-Jew, the Taz wrote: Taz, Yoreh De'ah 151:5:
"The custom is to permit it in all cases"—the reason, as cited in the Beit Yosef in the name of the Sefer Mitzvot Gadol, is that nowadays the concern of testing the animal does not apply, because we are not so expert in voice commands, and also it is not our practice to lend or rent animals to idol worshippers.

That is, it seems that in both cases they are lenient regarding the decree because the reasons are less relevant to present-day reality, and therefore I am asking: why should we not also be lenient with the decree about grinding medicinal ingredients nowadays?

I do not have a good answer to that, and certainly not a general one. As I wrote to you, in principle even when the reason no longer applies, the decree is not nullified. But there are many exceptions. See the discussion in Guttel’s aforementioned book.