Elves and the halls of Mandos

Note 17 doesn't actually add too much. It just says that there is no mention of Finwe and Indis' estrangement in the earlier story given in MR. It also mentions a separation is implied in the final Quenta by the fact that Indis did not accompany Finwe when he went into banishment with Feanor. (Feanor's wife Nerdanel would not go to Formenos and asked leave to stay with Indis.)

The story of what happened to Finwe when he went to Mandos is discussed in HoME X a bit before this (in the Laws and Customs section). Miriel did not wish to return from Mandos and after a time, Finwe wanted to marry Indis. After much debate, the Valar decided to allow this on the condition that Miriel agreed to remain in Mandos forever, since an Elf cannot have two spouses who are alive at the same time. Miriel agreed. After Finwe was slain, he said he did not wish to return from Mandos himself, so Miriel was given a second chance to return. She was re-embodied but remained in the Halls of Mandos as a sort of handmaiden to Este: she helped with the weaving, since this was her great talent.

To address the original question, Namo (Mandos) decides how long a particular spirit remains in his halls. The length of time is dependent on that spirit's actions during its lifetime. We don't know about Fingolfin; he isn't mentioned as returning. Finrod is mentioned as having returned, but we don't know when this was. He was allowed to return because he was "good": he sacrificed his life so Beren could continue his quest.