I've delayed answering this question because I've been looking for an explanation of the symbolism behind the complicated manner of Lleu's death. I still haven't a satisfactory explanation from a reputable source, although I'm sure one exists, so this answer is incomplete. However, I think I've answered most of the original question, and I've taken a stab at the symbolism question. If anyone has any additional research, please let me know.
Why Lleu is so difficult to kill?
This story is a heavily modified version of the "Tristan and Iseult" archetype, in that there exists a king or ruler or giant who will die when their daughter is married. The ruler attempts to prevent his daughter from being married, but of course she eventually circumvents the ruler and get's married anyway, at which point the ruler dies. This story relates to death but also to rebirth; although the ruler dies, because the daughter marries the ruler's legacy continues, and the husband or son of the daughter replaces the ruler in the mortal world. (A slightly different version also exists: a prophesy says the ruler will die if he has a son, the ruler attempts to not have a son, he has a son anyway, and the son kills his father and takes his father's place.) Of the top of my head, Perseus, Tristian and Iseult, and Culhwch and Olwen are all examples of this type of story.
To quote from Roger Loomis' review of Math Vab Mathonwy, An Inquiry into the Origins and Development of the Fourth Branch of the Mabingogi (I didn't want to read the entire book):
The chief discovery is the ultimate dependence of Math upon the Irish myth of Balor and Lug for its main outline (pp. 65-112). Significant is the fact that this myth, surviving most clearly in folktales taken down in the nineteenth century, must have supplied the framework for a story composed as early as the twelfth century. It is a variant of the widespread formula of the King and His Prophesied Death (pp. 8, 366), of which the Perseus and Oedipus stories furnish familiar examples. The plot of the famous Irish story may be reconstructed as follows... Now the Welsh tale is remarkable in that it preserves practically every essential feature of this outline in the right order, but every feature has been recast several times, and all the original cohesion and rationale has been lost. Math is at pains to preserve the virginity of a damsel, but no reason is offered and the damsel is not his daughter. The damsel is ravished by stratagem, but nothing is said of her son. Instead, the woman who takes her place gives birth to a son, Dylan, and he promptly goes to the sea and, instead of drowning, swims like a fish. But it is not he who becomes the hero of the story but one who develops from the caul or placenta which Dylan's mother lets fall at the time of his birth. This caul develops into a second child, and upon him are sworn successively the three geasa that were sworn upon Lug: but it is his mother who swears them, and she obviously has no motive. The way in which she is induced to name him Lleu Llaw Gyffes is obviously reminiscent of the way in which Balor was induced to name his grandson Lug Lamfada. Finally, it is not Lleu's grandfather who is slain by a special spear, but Lleu's rival, Gronwy. Except that the order remains intact, the elements could hardly have been juggled more drastically.
A common feature in these types of stories is that the father-figure is very difficult to kill, and can only be killed in strange and esoteric ways. Here's an example from Culhwch and Olwen, a story from the same tradition as Math. (The story of Balor and Lug, upon which Math is based upon, also includes a father figure who is hard to kill).
And the third day they returned to the palace. And Yspaddaden Penkawr said to them, "Shoot not at me again unless you desire death. Where are my attendants? Lift up the forks of my eyebrows which have fallen over my eyeballs, that I may see the fashion of my son-in-law." Then they arose, and, as they did so, Yspaddaden Penkawr took the third poisoned dart and cast it at them. And Kilhwch caught it and threw it vigorously, and wounded him through the eyeball, so that the dart came out at the back of his head. "A cursed ungentle son-in-law, truly! As long as I remain alive, my eyesight will be the worse. Whenever I go against the wind, my eyes will water; and peradventure my head will burn, and I shall have a giddiness every new moon. Cursed be the fire in which it was forged. Like the bite of a mad dog is the stroke of this poisoned iron." And they went to meat.
Since Lleu takes the role of the father figure in this rearranged story, it also makes sense that he is hard to kill.
An attempted explanation of the symbolism behind Lleu's position:
The easiest answer is that the position is the answer to the riddle: he can't be killed "inside or outside," so he straddles both worlds and becomes vulnerable. However, this doesn't explain why he needs to do this.
I've done quite a bit of research into the symbolism of Lleu's death, and having read several sources and talked to a few experts, I'm forced to conclude that no agreed upon explanation exists. However, I have seen a few theories, which I outline below:
It's important to know that Lleu seems to be associated with the sun. According to Queen of the Night: Rediscovering the Celtic Moon Goddess:
In the Mabinogi, Lleu Llaw Gyffes (Bright One of the Skillful Hand) is transformed into an eagle, residing for a period of time in the top of an oak tree. The eagle and the oak are symbols of a solar or celestial deity.
I found an article that claims that the position that Lleu can be killed corresponds to something related to astronomy (the hut to the sky, the tub/well to the milky way, the goat represents Capella, and Lleu to the constellation Perseus [Perseus seems to have a foot in the milkyway]). The article seems legit, and is definitely worth considering, but I worry that it doesn't seem to be verified by any academics. Here's a picture of the constellation Perseus (from AstroPixels.com):
Another explanation (found in an article about Shakespeare!) ties the age of the spear to the creation of a new year (and perhaps to rebirth):
Blodeuwedd and her lover planned to slay her husband, the Welsh hero Lleu Llaw Gyffes; but he could only be killed by a spear forged "in a year of Sundays"; and so Lleu was struck down exactly one year after the plan was conceived. The death of the year equated the death of the husband; either event signals or is signaled by the Queen's or Lady's attachment to another man.
(Aguirre, Manuel. Life, Crown, and Queen: Gertrude and the Theme of Sovereignty.)
And as a bonus, here's an article that elaborates on the connection between names, transformations, and the difference between Lleu's and Blodeuwedd's metamorphoses:
The metamorphoses of Lleu and Blodeu(w)edd confirm the theory that, in the Fourth Branch, names function to secure both body and identity. The scene in which Lleu spontaneously transforms into an eagle and the earlier scene in which he is named are linked in such a way that his name emerges as a crucial guarantee of human embodiment; by contrast, Blodeu(w)edd is renamed so that her metamorphosis can never be undone. Having persuaded her husband, Lleu, to reveal how he may be killed, Blodeu(w)edd has Gronw ambush Lleu while he is demonstrating to his wife exactly the point at which he is at his most vulnerable. Gronw’s ambush prompts Lleu’s transformation: “Then Gronw rose up from the hill that is called Brynn Cyfergyr, and he got up on the end of one knee, and cast the poison-spear, and hit [Lleu] in the side, so that the spear shaft leapt from him and the head stayed in him. And then he went flying up in the shape of an eagle, and gave a terrible cry” (Williams 1951, 88). Although the text gives more details about the spear thrust than the metamorphosis, Lleu’s transformation clearly results from the entry of the spearhead into his body. This passage mirrors the episode in which the young Lleu is named. In this scene, disguised, Gwydion brings the boy to visit Aranrhod; when Aranrhod sees the boy hit a wren with surprising precision and comments on his dexterity, saying, “it is with a skilful hand Lleu can be killed only by a single blow from a spear made only during Sunday mass over the course of one year, and he must be neither inside nor outside, neither on horseback nor on foot; the inventive solution to this riddle involves Lleu’s standing with one foot on the edge of a roofed outdoor bathtub, the other on the back of a goat. The actions in both scenes are also described by identical verbs, bwrw (to throw, cast) and medru (to strike). Sheehan that the fair one hit it [ Ys llaw gyffes y medrwys y lleu ef ]” (Williams 1951, 80), Gwydion manipulates her comment to provide a name for his nephew, Lleu Llaw Gyffes (Fair-haired One of the Skilful Hand). Lleu’s name is also the key to his metamorphosis’s reversal. When Gwydion finds the eagle perched in a treetop, he sings three englynion , each time naming Lleu, until the eagle descends from the tree and alights on Gwydion’s knee; Gwydion is then able to strike him with the hutlath “so that he was in his own shape [ rith ]” (Williams 1951, 90). Lleu’s metamorphosis is consistent with those examined above, in that the narrator does not refer to Lleu by name until he is again in his own, human body. Lleu cannot regain his body until he regains his name: it is Gwydion’s ritualized poetic naming of his nephew together with a tap of the hutlath that restores Lleu to human form. Gronw’s skillful striking of Lleu causes Lleu to lose both name and form by reversing the subject positions (active/passive, striker/struck) of the scenario whereby he gained his name
(Sheehan, Sarah, Matrilineal Subjects: Ambiguity, Bodies, and Metamorphosis in the Fourth Branch of the Mabinogi)