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It's obviously a matter of opinion, but I happen to both agree and disagree. If you think about it, the name Celeborn is, after all, Sindarin (better than the QuenyaTeleporno, I think!), and should be pronounced and thought of as Sindarin; not as English! If the name were of English origin, the name would obviously be pronounced as an 's'. But if thuoght of as an Elvish word, Celeborn would sound silly if said with an 's'. the 'k' is much more appropriate.
And as for the 'Mannish' sound, I might agree with you if more "uncouth" letters like 'g', 'j' or 'ch' were included in the name. But in many words, such as the Khazad word khelek for 'glass', the 'k' is not only appropriate but adds to the effect that the word portrays.
I somehow always thought of Cirdan as a K. Celeborn, Celebrian etc on the other hand I originally thought of as a S until I read the appendix. That took some getting used to. At least I got them right in the Sil!
I always thought of it as Seleborn and Sirith Ungol. Maybe I'm wrong (and apparently I am), but I read the books three times thinking that so i'm just gonna go on thinking that way!