[Formendacil]

If it is from Shibboleth, then it's almost certainly acceptable Sindarin, since the Shibboleth is quite late--mid-1960s, though I'm hesitant to quote a year, but even if it were from the 1950s, it would still be post-publication of the LotR, and if my memory is correct, it would be contemporaneous with the 2nd Edition of the LotR, in which Tolkien did tinker a bit with the Elvish in the book--though my impression (I am not a Sindarin expert) is anything post-LotR is probably going to hold sufficient water. It's when you start going into the pre-LotR versions of things, especially back at the Book of Lost Tales end, that you run into problems.

(That said, I have no memory of Gil-galad's mother's name in the Shibboleth, but it's been a while since I read that volume... I'm rereading the Book of Lost Tales right now, so "Meril" reminds me more of the Queen of Tol Eressëa in that text--but given how many other very early names stayed in circulation (Beren, Manwë, Ulmo, etc) or came back (Legolas), it would not surprise me to see Tolkien reusing the name--and if it had that much staying power in his mind, I really wouldn't worry about it being acceptable Sindarin.)

EDIT: Digging out my copy of The Peoples of Middle-earth (the HoME volume including the Shibboleth), I don't see "Meril" in the index--and the HoME indices are generally excellent. A cursory, but I think accurate, skim of the text, especially the commentary towards the end where Christopher Tolkien goes through the history of Gil-galad's parentage (or lack thereof) reveals nothing either. If Meril was ever put forth as the name of his mother, it would not seem to have been the Shibboleth. My suspicion is it might be fanon, though of a deft sort.