Question

There are halakhic legal fictions that are permitted ab initio (“one may use a legal fiction regarding second tithe” by selling it in order to avoid the extra fifth), and there are problematic ones (bringing produce in through enclosures in order to be exempt from tithes). The examples are from here. And then there are the sale of firstborn animals, the sale of leavened food, the sale permit for the Sabbatical year, the business partnership arrangement to avoid the prohibition of interest, the prosbul, the eruv tavshilin, and so on.
The Rabbi wrote that in formal laws one can use a legal fiction, because halakhic formalism contains everything, and whatever is permitted is permitted. But in consequentialist or moral laws one does not do so. (I couldn’t think of an example. Maybe indirect damage? There one is liable in the judgment of Heaven, and that seemingly means that the legal fiction fails and it is still forbidden.) Quoting from the responsum here:
[It really would have been proper for me first to gather as many examples as possible of permitted and forbidden legal fictions in the Talmud and among the halakhic decisors before approaching the question (and maybe the answer would become clear just from seeing the examples), but from passing remarks—more precisely, the wing, breast, and thigh—I understand that the Rabbi has already sorted and arranged the legal fictions, each woman under her banner with the signs of her fathers’ house; maybe there is an article about this? (I have some memory of having read such a thing once, but I couldn’t find it now by searching the site.) I really am asking from a very high-level perspective, without sticking to the details, because I’m not expert in them. If that lack is critical to the answer, then I’ll go and change this chapter.]

My questions:

A. Where does the assumption come from that halakhic formalism contains everything? After all, it is a frequent refrain of yours that the positivist pretension in law—that one can formulate the exact intention in words without leaving lacunae or room for interpretive freedom—is an illusion. I understand that the meaning is that where we have no access to the spirit of the matter, we are forced to make do with formalism. Is that indeed so? And is this only in cases of legal fiction where it is easy for us to see how the Jewish law could have been formulated so as to forbid this loophole?

B. After the distinction between Jewish law and morality, why can’t one also use legal fiction in laws that do have a “spirit”? The legal fiction will work on the halakhic plane, and the moral problems will remain exactly as they are. From the fact that there is a distinction, I understand that the spirit here is a distilled halakhic spirit (the rationale of the verse—what this law is trying to achieve) and not a moral spirit, and if so the distinction between formal laws (where we failed to decipher the spirit, and therefore ignore it and make do with formalism) and laws that are not only formal (where we did decipher the spirit, and therefore are careful not to circumvent it) is less clear to me.
And also, is halakhic intuition really too weak to find a spirit for the extra fifth in redeeming second tithe, or for a mother animal and its young that fell into a pit on a Jewish holiday (where one uses a legal fiction and brings them both up)? (Not to mention interest, prosbul, second tithe, leavened food, and eruv tavshilin.