‘O Light of Pleiades’ [Translation]

How would one say ‘O Light of Pleiades’ in Sindarin? is it ‘A Gil-Remmirath’? Would there be any morphing of 'lr' into 'll'? Gillemmirath?

The simple answer to your question: no, it shouldn't.

The simple genitive in Sindarin is shown by word order, not by linking/mutation/assimilation. As we see in the Moria Gate Inscription, "Ennyn Durin" does not make use of a hyphen nor assimilation, and appropriately it is of a noun and a proper noun, like your translation.

Best wishes,

Ríon Gondremborion