The verb N. hadh- “sit” appeared in The Etymologies of the 1930s under the root ᴹ√KHAD of the same meaning, but the verb and its root were revised to N. haf- and ᴹ√KHAM (Ety/KHAM; EtyAC/KHAM).
Neo-Sindarin: In Tolkien’s later writings √KHAD “sit” was restored, so I would use ᴺS. hadh- “to sit” for purposes of Neo-Sindarin; compare the 1969 verb S. hadhwa- “to seat” < ✶khadmā (PE22/148).
Conceptual Development: The Gnomish Lexicon of the 1910s had G. hag- “sit, sit down” (GL/47).
A verb appearing as har “stand” on the back of one version of Thror’s Map from the 1930s (AotH/56). It might be related to ᴹQ. thar- “stand” from 1948 (PE22/126).
Neo-Sindarin: Since the later Elvish root for “stand” is √TAR, I would update this verb to ᴺS. tar- as suggested by Elaran.