The root ᴹ√SAM “unite, join” was a later addition to The Etymologies of the 1930s with the derivative ᴹQ. samnar “diphthongs” (Ety/SAM). There is also evidence for it in the word ᴹQ. sampane “combination” as in ᴹQ. Lámasampane “Combination of Sounds”, a term used in the first version of the Tengwesta Qenderinwa (TQ1) from the 1930s (PE18/40), and again in the second version (TQ2) from around 1950 (PE18/90). In an isolated note from the late 1930s, Tolkien gave ᴹ√kam “bind, join” as a replacement for √sam along with a new word ᴹQ. okamna “diphthong” (VT44/13), but given the reappearance of Q. sampanë in TQ2 this may have been a transient idea, and in any case Tolkien used the word Q. ohlon for “diphthong” in the 1950s and 60s (VT39/9; VT48/29).
In notes grouped with Definitive Linguistic Notes (DLN) from 1959, Tolkien gave √SAM as the basis for Elvish verbs for “to have”, with Q. samin and S. sevin “✱I have” (PE17/173). Whether this was connected to 1930s ᴹ√SAM “unite, join” is unclear. In notes associated with the 1959-60 essay Ósanwe-kenta, Tolkien gave the root √SAM with the gloss “mind, think, reflect, be aware” (VT41/5), but in later writings he used √SAN for “think, use mind” instead (PE22/158); see that entry for discussion.
Neo-Eldarin: For purposes of Neo-Eldarin, I’d use √SAM = “have”.
A root appearing in some late notes on verbs from around 1969, with the sense “possess, own, keep” (PE22/151). It seems to be a restoration of a much earlier root from the 1910s, which appeared as ᴱ√AW̯A in the Qenya Lexicon with derivatives having to do with wealth, such as ᴱQ. ausie “wealth” and ᴱQ. aute “rich” (QL/33), as well as Gnomish and Early Noldorin cognates G. avos “wealth” (GL/20) and ᴱN. awes “rich” (PE13/137). This root is a good example of how certain linguistic ideas could lay dormant for many years in Tolkien’s writing, only to reemerge much later. This makes it difficult to say for certain whether Tolkien really abandoned a particular idea.