A word for “throne” in Late Notes on Verb Structure (LVS) from 1969, equivalent to Q. tarhanwa, with various Sindarin forms arahand, arahaðm, arahaðw and archaf (PE22/148). The initial element is ar(a)-, which was elsewhere glossed “noble, royal, high”. The second element is derived from √KHAD “sit” (PE18/95). The variant with -hand seems based on (unattested) primitive ✱✶hand(ǝ); compare this to deleted {N. hand} “seat” from The Etymologies of the 1930s (EtyAC/KHAM). The variants with -haðm and -haðw are clearly based on primitive ✶khadmā “seat” appearing on the same page in LVS (PE22/148), with -haðm being the more ancient form.
The fourth form with -chaf may also be a variant derivation of the ✶khadmā, since lower on the page Tolkien proposed dm > v as a Sindarin phonetic development. This development is inconsistent with Tolkien ideas elsewhere (generally dm > ðw > -dhu when final) and I think it was a transient idea.
Neo-Sindarin: For purposes of Neo-Sindarin I would combine the final element of the third form with the initial element of the fourth form to get ᴺS. archadhu “throne, (lit.) high seat” as the most phonologically plausible form.
Conceptual Development: The Gnomish Lexicon of the 1910s had G. doros “throne” as an elaboration of G. dorn “seat” (GL/30). Early Noldorin Word-lists of the 1920s had ᴱN. {aicod >>} turhod “throne”, a combination of ᴱN. tûr “power” and ᴱN. haud “seat” (PE13/154-155).
The 7th and 10th chieftains of the Dúnedain (LotR/1038). The initial element of the name is the prefix ar(a)- “noble” and its final element might be the lenitied form had of the noun sad “place”.
Conceptual Development: In the draft of Appendix A of the Lord of the Rings, the name of Arahad II was first given as Arangar (PM/211).