Os Moriquendi são definidos por linhagem ou mais genericamente?

Embora não seja exatamente claro, é mais provável que suas pressuposições estejam corretas.

Ao olhar em volta, não consegui encontrar muita coisa que discutisse se os títulos eram hereditários ou se eram baseados no nascimento de cada indivíduo. Alguns internautas são da opinião de que, no momento da destruição das árvores, o título se tornou um título de nobreza e era hereditário, no entanto, com a longa vida dos Elfos, pode ser confuso.

Um bom exemplo para apoiar seu ponto é Luthien. Embora seu pai fosse um Calaquendi, e sua mãe uma Maia, ela foi considerada entre os Moriquendi (indiretamente apoiada na seguinte citação)

he [Thingol] alone of all of the Sindar had seen with his own eyes the Trees in the day of their flowering, and king though he was of the Úmanyar, he was not counted among the Moriquendi, but with the Elves of the Light, mighty upon Middle-earth.
_{The Silmarillion - Of Thingol and Melian}

Dado que ele era o único Sindar que tinha visto a luz das árvores, pode-se presumir que Luthien (um elfo Sindarin) foi considerado entre os Moriquendi.

Os Calaquendi eram - no seu sentido mais simples - aqueles que tinham inicialmente visto a luz das Duas Árvores

These were the three kindreds of the Eldalië, who passing at length into the uttermost West in the days of the Trees are called the Calaquendi, Elves of the Light. But others of the Eldar there were who set out indeed upon the westward march, but became lost upon the long road, or turned aside, or lingered on the shores of Middle-earth; and these were for the most part of the kindred of the Teleri, as is told hereafter. They dwelt by the sea or wandered in the woods and mountains of the world, yet their hearts were turned towards the West. Those Elves the Calaquendi call the Úmanyar, since they came never to the land of Aman and the Blessed Realm; but the Úmanyar and the Avari alike they call the Moriquendi, Elves of the Darkness, for they never beheld the Light that was before the Sun and Moon
_{The Silmarillion - Of the Coming of the Elves and the Captivity of Melkor}

No entanto, o termo também foi usado para descrever todos os elfos que tinham vivido entre os Valar em Aman:

In those days Elves and Men were of like stature and strength of body, but the Elves had greater wisdom, and skill, and beauty; and those who had dwelt in Valinor and looked upon the Powers as much surpassed the Dark Elves in these things as they in turn surpassed the people of mortal race. Only to the realm of Doriath, whose queen Melian was of the kindred of Valar, did the Sindar come near to match the Calaquendi of the Blessed Realm.
_{The Silmarillion - Of Men}

Que é mais suportado no índice:

Calaquendi : 'Elves of the Light', those Elves who lived or had lived in Aman (the High Elves).
_{The Silmarillion - Index}

Assim, os filhos dos Noldor nascidos na Terra-média seriam todos do Moriquendi (Maeglin, Gil-galad e assim por diante), mas sendo ensinados pelos Elfos da Luz, excederiam em conhecimento e poder os Sindar e outros 'Dark Elves'.

A Guerra das Jóias também lança um pouco de luz sobre o assunto, embora de rascunhos anteriores, parece que o uso do Moriquendi se tornou uma referência a todos aqueles que não eram súditos de Elwe.

The Quenya forms were Kalaquendi and Moriquendi. The Kalaquendi in Quenya applied only to the Elves who actually lived or had lived in Aman; and the Moriquendi was applied to all others, whether they had come on the March or not. The latter were regarded as greatly inferior to the Kalaquendi, who had experienced the Light of Valinor, and had also acquired far greater knowledge and powers by their association with the Valar and Maiar.

In the period of Exile the Ñoldor modified their use of these terms, which was offensive to the Sindar. Kalaquendi went out of use, except in written Ñoldorin lore. Moriquendi was now applied to all other Elves, except the Ñoldor and Sindar, that is to Avari or to any kind of Elves that at the time of the coming of the Noldor had not long dwelt in Beleriand and were not subjects of Elwë. It was never applied, however, to any but Elvish peoples. The old distinction, when made, was represented by the new terms Amanyar 'those of Aman', and Úamanyar or Úmanyar 'those not of Aman', beside the longer forms Amaneldi and Úmaneldi. _{History of Middle-earth IX, The War of The Jewels - Quendi and Eldar}