Something strange

['Oma Tirea]

I see this popping up virtually everywhere here....

Tawtute becomes a lentitioned sawtute, but without the ay- prefix.  Is it because this is now a coined/contracted word for the plural of "Sky person," or is it simply improper Na'vi grammar?

Irayo in advance.

[wm.annis]

Tawtute becomes a lentitioned sawtute, but without the ay- prefix.  Is it because this is now a coined/contracted word for the plural of "Sky person,"

Right.  For the plural prefix (only — the dual me+ and trial pxe+ also cause lenition, but can never be dropped), if it causes lenition it can be dropped, leaving only the lenition in place as a sign that the word is plural.  So,

koren rule — ayhoren or horen rules
lì'fya language — aylì'fya languages (since l does not take lenition, ay must stay)

[kewnya txamew'itan]

Neither, it's correct na'vi grammar. If the nouns lenits then the ay+ prefix can be dropped, stating it explicitly is still allowed but in particularly informal situations might sound stilted.