In the Gemoro Kesuvos on Daf Zayin Omud Beys there is a Gantzer Shakler Vetaria between Rav Nachman and Rav Avohu about Darshening various Peshkim for various Halochos .
Lichoroh it looks in the end Az only Rav Nachman Darshens Moavi Veloy Moavis from the Possuk in Rus so Im Keyn does that mean according to Rav Avohu that Boaz Darshened incorrectly and he should not have married Rus or Keyn Zyn Boaz was correct and there is a Zar Min Halochoh but this was not the place for it.To me it does not look clear what Rav Avohu holds Legabay this.
There is no Machlokes between Rav Nachman and Rebbi Avahu whether we hold of the Derashah "Mo'avi but not Mo'avis". The only dispute is whether it was necessary to announce this Derashah in front of 10 people. When the Gemara states that Rebbi Avahu learned that Boaz took 10 elders "to Darshen Amoni but not Amonis etc.", this means to Darshen it in public. The Gemara explains that according to Rebbi Avahu the reason Boaz needed specifically to take elders is because the elders - i.e. Talmidei Chachamim - are the ones one has to publicise a Halachah in front of.
The Shitah Mekubetzes DH v'Hilchesa writes that Rav Nachman holds that since the Derashah "Mo'avi but not Mo'avis" is simple and obvious, it was not necessary to say it in front of 10. In this respect it was different from "ha'Mezakeh l'Ubar Kanah", which Rav Nachman agrees was necessary to say in front of 10, because the latter was not Pashut - in fact we do not Pasken like this. In contrast we certainly do Pasken that a female Moabite convert is permitted to a Yid. Rav Nachman holds that it was not necessary to say this Halachah in front of 10, because the very fact that Boaz got married to her was the greatest possible publicity.
The Sukas David #67 explans Rebbi Avahu's Shitah. Rebbi Avahu holds that "Mo'avi but not Mo'avis" is not simple. The proof for this is that the "Go'el" (Redeemer) of Rus, the closer relation than Boaz ("Tov," Rus 3:13), did not want to do Yibum with her, because he said (Rus 4:6) "Lest I destroy my inheritance". This was because the close relative, Tov, did not hold of the Derashah that permitted the Moabite women, so he held that Rus was forbidden to marry a Yid.
However everyone agrees that the Halachah does not follow Tov's apprehension, because Boaz possessed a tradition from Shmuel the Prophet, from Ramat, that because Mo'av is forbidden since they did not go out to meet Bnei Yisrael and offer them bread and water, it follows that this only applies to males, because it is anyway not the way of women to go outside and offer their hospitality. In addition Mo'av are forbidden because they hired Bilam against Bnei Yisrael, but again it was only the Mo'av men who hired him, not the women.
Rebbi Avahu agrees that Rus was permitted to Boaz, but since this was not a simple Din, he maintains that it should have been publicised in front of 10 people by Boaz, so that no one should suspect him of transgressing the Halachah.