It seems I caused some confusion, I should've elaborated more.
The reason I thought that the names were intended to be Sindarin is because of Ferenneth and Ferenthel (the latter is incorrectly formed though). If you want Ferenneth in Quenya, it would probably be Ferennette (here I should note that Quenya names are not my specialty).
Ferendir, Ferendis, Ferenion and Fereniel can work as either Quenya or Sindarin without changing their forms.
As for the words for "beech":
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In Quenya we only have feren.
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In Sindarin we have neldor, though its etymology is slightly dubious. But we also have fêr, though it is given as a more archaic word (supplanted by neldor).
Sindarin fêr is a direct cognate of the Quenya feren, both coming from the root √PʰEREN:
Quenya: ✶pʰeren > ɸeren > feren.
Sindarin: ✶pʰeren > ɸeren > ɸere > fere > fer > fêr.
The historical development of the Sindarin word shows the loss of the final nasal consonant, which then allows for the vowel that preceded it to be lost as well. This means that in a compound, where that consonant is not final, fêr would take the form feren-, hence why the names Ferendir, Ferendis, Ferenion and Fereniel are essentially the same in both Quenya and Sindarin.