Did Tomoe ever find out that the Yukiji that he fell in love with was Nanami disguised as Yukiji?
Tomoe realized that the woman he fell in love in the past was never Yukiji, but really Nanami, who was actually the descendant of Yukiji and her child. The two then start a relationship to keep their promises of the past. It is explained in Kamisama Kiss: Kako-Hen that the “Yukiji” who Tomoe met was Nanami who was travelling back in time to save Tomoe. If Yukiji were to ever meet Tomoe, she’d try to kill him. Yukiji absolutely detests youkai, so there is no way that she’d ever love him as well.
Tomoe has been in love with Nanami this entire time, he just believed that she was Yukiji.
Source: https://kamisamahajimemashita.fandom.com/wiki/Tomoe