Shock! Horror! Feanor does what a Vala says...

[Elmo]

Why do you think Feanor answered the summons of Mandos and entered his Halls after he was slain? He did not have to and he seemed completly set against the Valar at the time of his death. He could have chosen to drift about like other the other houseless spirits. I'm very suprised he did not want to stay around and see how his sons got on. Or maybe he would have yearned to have another glimpse of the silmarils that he loved so much?

Perhaps this shows that Feanor had given up his fight against the Valar but then again he compelled his sons to remember the oath of hatred that they had took. Maybe he was forced to by Mandos. His thinking being about what hi-jinks Feanor could get up to without a body :D

[Raynor]

Maybe he was forced to by Mandos.That cannot happen
It is summoned; and the summons proceeds from just authority, and is imperative; yet it may be refused. Concerning those who refuse the summons, it is stated:
It was less frequent [to refuse the summons of Mandos], however, in ancient days, while Morgoth was in Arda, or his servant Sauron after him; for then the fea unbodied would flee in terror of the Shadow to any refuge - unless it were already committed to the Darkness and passed then into its dominion. In like manner even of the Eldar some who had become corrupted refused the summons, and then had little power to resist the counter-summons of Morgoth.So, according to the bolded parts, a fea who refuses the summons of Mandos has three 'roads':
- to flee in terror
- to pass into the dominion of Darkness, if the fea was already commited
- to use the "little power" to refuse the counter-sammons

Although I believe that Feanor had enough inner power to refuse Melkor, even after his/kin-slaying, I believe he was still good enough to answer Mandos. In the Silmarillion, it is stated that although Feanor was ensnared by Melkor, he didn't receive counsel from him or held converse with him, so I still bet on Feanor.

[Lord Melkor]

Hmmm, are you sure these three roads are correct, Raynor? If I read the quote you posted correctly it discusses several things.

1. Why refusal of the Summons of Mandos occurred less frequently in ancient days:

It was less frequent [to refuse the summons of Mandos], however, in ancient days, while Morgoth was in Arda, or his servant Sauron after him; for then the fea unbodied would flee in terror of the Shadow to any refuge

If I read this quote correctly I take it to mean that the unbodied fea would flee the terror of the Shadow to the safety of the Halls of Mandos.

2. Exceptions to the previous rule:

...unless it were already committed to the Darkness and passed then into its dominion.

Unbodied fea who were committed to Darkness would refuse the summons of Mandos and would instead fall under the dominion of the Shadow.

3. What happened to corrupted Eldar.

It is important to note that the previous lines primarily discussed what happened to the fea of the Avari, the Elves that refused to go West, which becomes clear if we look at the preceding sentences.

Among those who refused the summons (or rather the invitation) of the Valar to Aman in the first years of the elves, refusal of the summons to Mandos and the Halls of Waiting is, the Eldar say, frequent. It was less frequent [to refuse the summons of Mandos], however, in ancient days, while Morgoth was in Arda,....

Therefore the line:

In like manner even of the Eldar some who had become corrupted refused the summons, and then had little power to resist the counter-summons of Morgoth.

is not really a 'third option' but is simply to illustrate that even one of the Eldar would not be able to resist the counter-summons of Morgoth if he refused the summons to Mandos, just like the Avari from the preceding sentence.

[Anguirel]

I have a rather non-technical suggestion.

Perhaps he wanted to "see" his father and mother again, reunited.

I like to hope that Feanor's family, sans Maglor, got happiness of some kind on their union within the Halls.

[Raynor]

If I read this quote correctly I take it to mean that the unbodied fea would flee the terror of the Shadow to the safety of the Halls of Mandos.While the halls of Mandos are a likely destination, they are not the only possible one - the qualifier is inclusive "any" - not exclusive, such as "only".
Unbodied fea who were committed to Darkness would refuse the summons of Mandos and would instead fall under the dominion of the Shadow.We're in agreement here.
is not really a 'third option' but is simply to illustrate that even one of the Eldar would not be able to resist the counter-summons of Morgoth if he refused the summons to Mandos, just like the Avari from the preceding sentence.The text does not exclude power to resist counter-summons completely. This remains a possibility - theoretical for the weak.

[Elmo]

Just a random question, what does Morgoth do with the Elven spirits he summons? Turns them into orcs, zombies...

[The 1,000 Reader]

Refering to Maglor, where was the beach he wandered on? If it was in Beleriand, wouldn't he have been killed in the sinking? Also, if he didn't eat from depression, could he have starved?