Let's start from the second question.
It is true that Tolkien variously mentioned 5-day, 6-day, and 7-day weeks.
The word otsola (Quenya) comes from very early drafts, 1910s-1920s. It is conceivable that at the time Tolkien intended the 7-day week to be the only format. That obviously changed later.
The word lemnar (also Quenya) appears much later, in 1938. It seems to refer specifically to the Valian week, at the time of the Two Trees.
And then we have enquie (likewise Quenya), referring to a 6-day week, mentioned in LotR.
Now putting the words aside, here's what we know about weeks lore-wise:
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At the time of the Two Trees in Valinor, the Valar may have used a 5 day week as mentioned above.
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The traditional Elvish calendar after the creation of the sun has 6 day weeks, see Appendix D.
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The Númenóreans introduced the 7 day week, later also used in Gondor and Arnor and later still in the Shire Reckoning.
Which boils down to this: Elves in Aman probably used 5 day weeks before the destruction of the trees; afterwards, Elves used 6 day weeks; humans later used 7 day weeks.
Now on to your first question - you use whichever language is actually being spoken. If someone is speaking Sindarin, they would obviously use the Sindarin names of the days. But yes, it is true that in later Ages Sindarin becomes much more frequent in daily use than Quenya. Either way neither set of names is "more important".
The 6-day system (enquie):
(Using a Quenya - Sindarin format)
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Elenya - Orgilion
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Anarya - Oranor
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Isilya - Orithil
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Aldúya - Orgaladhad
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Menelya - Ormenel
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Valanya/Tárion - Orbelain/Rodyn
The Númenórean 7-day week (differences in bold):
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Elenya - Orgilion
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Anarya - Oranor
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Isilya - Orithil
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Aldea - Orgaladh
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Menelya - Ormenel
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Earenya - Oraearon
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Valanya - Orbelain
And we actually also have names for the 5 Valian days (all in Quenya):
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Ar Manwen
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Ar Ulmon
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Ar Veruen
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Ar Fanturion
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(Ar?) Nessaron/Neldion
Technically we also have draft-Sindarin names for these but I am too lazy to update them now.