Hi. I never understood how Chazal ever had a hava amina about the Kusim being kosher for any purpose when they both had a safek geyrus (the geyrus with the lions according to what I saw in Meam Loez was only for 7 mitzvos Bnai Noach) and in any case they did not keep halacha properly. What is the answer to this puzzle? Even if some were megayer le-halacha, if they thereafter did not keep halacha properly, and were kofer in Torah she-be-al Peh, then why should there ever be such a hava amina?
I am not sure what the context of the Me'am Lo'ez was, but it seems that he is explaining the opinion who maintains that Kutim are "Gerei Arayos," i.e. their Geirus was not acceptable. According to the opinion that they were Gerei Emes, they accepted all of the Mitzvos.
Even if they did not observe some of the Mitzvos, since they accepted the Mitzvos (and at the time they accepted them, they intended to observe them), they and all of their decendants are considered Geirim.
An interesting point, though, is that the Rambam (in Perush ha'Mishnah and Mishnah Torah in countless places) observes that because later generations of Kutim were observed to serve idols, we indeed can consider them mid'Oraisa as *non* converts, and their conversion is (retroactively?) not valid, as you suggested.