Still, why couldn’t the Ri give a case nogeia to this ger, such as if he is mekadesh his father’s wife? The Ran and Ritva do, in fact, give such a case.
Or worse, that if his brother is a real brother from his mother that we should be Choshesh that he might do Yibum which would be an Erva. Why does Tosafos only bring up a Zekuka marrying out?
As for your Kasha, why should we care about him being Machmir in vain? Tosafos brought up an issue of where him being a full Yisroel would bring Issurim. And as for my Kasha, Tosafos’ doesn’t want to add more people to the story (which makes is less likely) in order to bring out the point.