Canonical value of the Shibboleth of Fëanor?

[Maédhros]

I was reading this most interesting discussion about the "High Kings" of the Noldor, and the subject of canonicity of the Shibboleth of Fëanor came up.
There appear to be certain "contradictions" in the Shibboleth that go against those in the Silmarillion and in Morgoth's Ring.
Examples are:

The death of Míriel Þerinde - death of an 'immortal' Elda in the
deathless land of Aman - was a matter of grave anxiety to the Valar, the first presage of the Shadow that was to fall on Valinor. The matter of Finwë and Míriel and the judgement that the Valar after long debate finally delivered upon it is elsewhere told. Only those points that
may explain the conduct of Fëanor are here recalled. Míriel's death was of free will: she forsook her body and her fëa went to the Halls of Waiting, while her body lay as if asleep in a garden. She said that she was weary in body and spirit and desired peace. The cause of her weariness she believed to be the bearing of Fëanor, great in mind and body beyond the measure of the Eldar. Her weariness she had endured until he was full grown, but she could endure it no longer.

Fingolfin had prefixed the name Finwë to Ñolofinwë before the Exiles reached Middle-earth. This was in pursuance of his claim to be the chieftain of all the Ñoldor after the death of Finwë, and so enraged Fëanor that it was no doubt one of the reasons for his treachery in abandoning Fingolfin and stealing away with all the ships.

The first one seems in direct contradiction with both the Silmarillion
and the Later Quenta Silmarillion in Morgoth's Ring.
The second, uses the new word "chieftain" which i believe is absent in the Silmarillion and the Later Quenta. What I find contradictory is that why would Fingolfin said that Fëanor would lead and he would follow him, and then pursued the kingship of the Noldor.

I wonder, is the Shibboleth can be regarded as the author's last view on this or it's something entirely different. Or was it entirely a historical phonology essay? If it turns to be of real canonical value, then must the Sil be adapted to conform to these changes.