Quenya adjective meaning “heavy” attested only as lungu- in the compounds Lungumá “Heavyhand” and lungumaitë “heavy-handed” (VT47/19, PE17/162). Given its stem form, it probably developed from primitive ✱✶lungŭ, which would be ✱lungo in Quenya since [[p|short final [i], [u] became [e], [o]]] in Primitive Elvish. This is consistent with its Sindarin cognate S. lung.
In one place, an earlier form of this word ᴹQ. lunga was glossed “fraught” in the phrase “fraught with sorrow” (PE22/124), as in “heavy with sorrow”. This indicates this word could be used in the metaphorical sense of “heavy” as well as its physical sense.
Conceptual Development: In the Qenya Lexicon from the 1910s, the word for “heavy” was ᴱQ. talka “heavy” from the root ᴱ√TALA “support” (QL/88). In the (Early) Noldorin Dictionary from the 1920s, Tolkien introduced the form ᴱQ. lungo “heavy” (PE13/163), but towards the end of that decade he used ᴱQ. lunga in notes associated with the Oilima Markirya poem (PE16/75).
ᴹQ. lunga reappeared in The Etymologies from the 1930s, this time as a derivative of ᴹ✶lungā, consistent with the a-affection in its Noldorin cognate lhong (Ety/LUG¹). At this point in time, Mablung was Doriathrin/Ilkorin rather than a Noldorin name (Ety/MAP), so there was no conflict with that name.
After Tolkien abandoned the Ilkorin language, Mablung would have become a Sindarin name, and Tolkien needed a new etymology for it. Judging by its later Quenya cognate Lungumá (VT47/19), it seems that Tolkien revised the primitive form of this word from ✶lungā to ✶lungŭ, as described above, possibly a restoration of its etymology from the 1920s. This meant there was no a-affection in the Sindarin development, making S. lung the Sindarin form of the word.
Neo-Quenya: Some Neo-Quenya writers (including myself at various points) prefer the earlier adjective for “heavy”: ᴹQ. lunga (Ety/LUG¹), since (a) it is directly attested and (b) has an obvious plural form lungë. Unfortunately, this earlier adjective is not compatible with S. lung. As such, I currently prefer Q. lungo, and would assume it has a plural form ✱lungwi similar to nouns like ᴹQ. ango (angu-), pl. angwi (Ety/ANGWA).
lunga adj. "heavy" (LUG1). Curiously, the variant lungu- appears in certain compounds; see Lungumá, lungumaitë.