Regarding the Gilgul, Explaining The Concept of Double Gilgul: There are two other parts of reincarnation (gilgul). 1) The first is one who in his first life coming to this world (e.g., when he is a 'new neshama'), he is able to merit and obtain his Nefesh, Ruach, and Neshama [in one lifetime, through his merits and performance of the Mitzvahs that are relevant to him]. But [in that lifetime] after he makes a sin ('chet') and he blemishes them (in that first time he has come to this world).
Here, when that person returns to reincarnate, to rectify [what he sinned or blemished in his first lifetime], he is unable to receive in this reincarnation his "NaRaN" (Nefesh, Ruach, and Neshama) at once. [This is because even if he rectifies his Nefesh in that life, his blemished Ruach (from the sins of the previous life) and blemished Neshama cannot rest on a perfected Nefesh). He can also rectify his soul as we described (at the end of Ch. 3) by saying as he lays down on his bed the passuk, "נפשי אויתיך בלילה וכו'."
The second way 2) is via one who in his first life, he did NOT merit anything other than his Nefesh alone [meaning, he never received his Ruach or his Neshama to blemish them]. This person [with only his Nefesh] sinned and damaged [only] his Nefesh [through the sin]. When he reincarnates [to fix the Nefesh, and he fixes it], he is able to receive his "NaRaN" (his Nefesh, Ruach, and Neshama) in this gilgul itself, since he did not damage his Ruach or his Neshama [in the previous life, because he did not receive them in that life].
Therefore, he can now come with the Nefesh after he has rectified it *as if* it was his first life. As it says with this, "אז זכה יתיר וכו' כנז"ל." However, in the first time that he came to this world, if he came with them all (he receives each of his Nefesh, Ruach, and Neshama, and he blemished them all, [in his next life,] even if he rectifies his Nefesh, how can a rectified Nefesh be a chariot ('merkava') for a blemished Ruach?
Similar question with a Neshama. In the case of when in his first life he only blemishes his Nefesh [because he did not merit to receive the Ruach or Neshama], [and here in this same first life he rectifies that blemish, (I am guessing through Teshuvah,)] he is able to receive all three souls in that same lifetime, as we described.