Then why is it not spelled Liwa? We already have the sound "ee" and it's "i". I am a linguist in training and this bothers me on a deep level. Letters and their sounds must have clear distinctions or it doesn't make sense as a communicative language in-universe. Plus, the E being "ee" sounds very English (mostly American) to me.
Here are two guides on pronouncing Maori by the way. While Matoran is not exactly Maori, it is a language that had big influence on Matoran, and I think it should factor into this decision:
Here it is never "ee" for example. In one of the pages they write it a little closer to an i sound than "eh", but not quite "ee"
Also, another Polynesian language, Hawaiian,pronounces their e as "ey" or "eh": https://www.omniglot.com/writing/hawaiian.htm as evidenced by this IPA (International Phonetic Alphabet) soundboard: http://www.ipachart.com/ The sound is classified as [e:], but that is simply the symbol "e" in the IPA.
LEE-wah seems like an American transliteration, translation, not sure how to put it, of the Matoran language word. But I think we need to have a language system that works properly instead. Outofgloom/Tolkien's big fan language project seems to be what I'm thinking of. It has internal logic to things such as pronuinciations, as well as grammar (although this part for the most part is derived from very little evidence, but still hold water I think). And that man has a Phd in linguistics if I am not mistaken. Pretty impressive.
Anyway, got off topic there for a moment. This is my case for the "e" in most instances being pronounced "eh" or "ey". And it being LAY or LEH, for both Lewa, Le-Matoran, Le-Koro, Le-Wahi etc.