Yevamos 77a - Converting a Moavi

It seems from the Mishnah and Gemara, and is explicit in the Rambam and Chinuch that there is no problem to convert a moavi man, just that if converted he is assur b'khal Hashem. This is muchrach by Mitzri, since the only way to have a mitzri sheini and shlishi, is that after conversion of a mitzri rishon, the child will be a sheini and their child a shlishi [As a side point, the Tosafos Yeshanim seems to understand that the issur of mitzri/edomi amoni/moavi, apply only after conversion. Based on this he explains the gemara's question on shlomo hamelech, how can he marry bas pharoh since she was a mitzri rishon. Meaning, if she was a goy, there is no question bec. the issur doesn't apply, but if she converted then there is an issur].

Now, the maharsha explains that the reason why do'eg was threatened that if he doesn't accept the halacha of "moavi and not moavis", he would be killed by sword was mida k'neged midah, since he was trying to kill with the sword. Do'eg claimed that an amonis conversion is invalid, in which case the marriage of Bo'az to Ruth was kanaim pog'im bo since she was a non-jew and they deserve to be killed like by pinchas. I really don't understand the maharsha, there is no question that Ruth was a Jew, the only question is whether she was mutar b'khal hashem?