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

[Raynor]

Maybe he was forced to by Mandos.

That cannot happen

Originally Posted by Of rebirth and of other dooms of those who go to Mandos, Later Quenta Silmarillion, HoME X

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:

Originally Posted by Of rebirth and of other dooms of those who go to Mandos, Later Quenta Silmarillion, HoME X

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.