Considerations of Evil Spirits and Leprosy in Halacha

Can we allow fish with meat according to the same consideration you mentioned?
Incidentally, I looked for the corresponding ruling in the Aruch Shulchan and found that it actually agrees with the Shulchan’s opinion and prohibits:
Aruch Shulchan Orach Chaim, section 4, section 15:
And he shall not touch it with his hand before putting it into his mouth, or into his nose, or into his ears, or into his eyes

What did you mean when you mentioned the Aruch Shulchan Yom Kippur?

Regarding fish, in principle yes. Although everyone is really strict about this and it should be discussed from a custom perspective.
Regarding the Aruch Shulchan, I don't have access to it at the moment. I remember about unholy water.

Regarding fish,
Isn't it known that a custom that was practiced because they were mistaken and thought that this was the law, and the law is not so – can be permitted without a permit, and is it reasonable to assume that most of the Jewish people practiced this custom because they were mistaken and thought that there was a danger in it and that this was the law, and therefore wouldn't it be possible to permit it without a permit? What if you were to say that the custom began as austerity and Hasidism, such as: “Rabbi Zira said: The daughters of Israel have become so strict on themselves that even if they see a drop of blood the size of a mustard seed, seven clean ones sit on it.”, or like the evening prayer, but this does not seem to be the case here.

Regarding the Aruch Shulchan, I think you meant this:

Aruch Shulchan Yoreh Deah, Section 5
Section B
But if a person eats a deadly poison or is bitten by a snake or is bitten by a mad dog, and the like, it is permissible because of predation and prohibited because of danger to life. And did they not feel the need to trample a snake in our barn? 7:7 And perhaps you may ask, why did we consider open water to be the death knell of an animal? Isn't there also a danger for a person to drink open water in places where snakes are found, as is well-known throughout the Shas? Of course, if we saw some kind of smell in an animal after drinking it, we would prohibit it from a person because of the dangers to life, like the death knell of a person. But the one who permits it is because we do not see any smell in an animal after drinking it, and it is not blood for all that. We thought that we saw smell like blood and the rest, and now we permit it. But we did not permit drinking open water unless there was smell in it, and this is what we likened to the death knell of an animal, which is also permitted, according to the fact that the snake certainly did not drink from it. If it had drunk from it, some smell would have been evident in the body of the animal [19:7]:

Regarding the fish, you are right. That is why I only wrote that it should be discussed because of custom, not because it is obligatory. The point to be discussed is exactly the one you raised.
But why do I still think it should be discussed? Because even if I think there is no danger in it and even if I think it is not a specific danger, I am not sure that I am right about it. And if they used to be strict about it, perhaps there is room for such a custom.

Regarding the Aruch Shulchan, indeed. You see that it changes the sage halacha in light of observations of what happens to people or animals in its environment. See a lesson by Rabbi Ovadia: http://www.ykr.org.il/modules/Videos/page/3299 See the Shulchan All the poskim talk about the change in circumstances (see, for example, his reference to the Maghrib, the Book of Reh, and many others). Incidentally, regarding water revealed in the Torah, the Shul, and the Arva, they write that it is because there are no snakes. In the quote you brought, he says something more radical: that we see among ourselves that it is not harmful.

There are many things these days that are of concern as to whether they are harmful to health or not (for example, consuming food products with preservatives or food colorings or artificial sweeteners, etc.), but even if there is health damage in these things, the reasonable person does not avoid them because of health concerns (unlike drinking kerosene or bleach). Therefore, would it not be correct to say that the prohibition of foods because of "and you were very careful for yourselves" is only in things that the reasonable person would avoid because of health concerns? Then we can say that someone who is very, very concerned about his health can also avoid eating fish along with meat (just as he can avoid preservatives and artificial sweeteners), but not that we would say that there is a halachic/moral obligation in this matter.

The difference is between things that the sages forbade and for which there is a prohibition and dangerous things that are not explicitly forbidden. In things that are explicitly forbidden, there is no room for consideration of how dangerous it is. Only if we have come to the conclusion that there is no danger at all and it is a mistake, can we cancel them. But in things that are not explicitly forbidden but in our assessment are dangerous, here there is room for discussion of how much risk there needs to be in order to prohibit. The rule among the poskim is “Shomer Fatayim G-d” meaning that what the world does not guard against, the halakha does not require us to guard against either. I do not think that eating food coloring, etc., is more dangerous than driving a car.

Regarding considerations of an evil spirit, see also:

http://onegshabbat.blogspot.co.il/2016/01/blog-post_29.html

Continuing with this issue, regarding washing hands with water between eating fish and meat, is it obligatory?

not.

And what about the last water, is it really mandatory?

Opinions were divided among the poskim, and even in the Shulchan Arash he presented both opinions (although in the Stima he writes that there is an obligation). The Mikal has something to rely on, especially since the taste for last water apparently does not belong today (salt of Sodom).