I agree with most of this. It's a good explanation. I was at first confused as to why Taichi would say "dad" during sexy time and after reading again I think that he did that on purpose, first to remind himself that he has to still be there for his dad and second because he was using it as a way to put some distance between Zama and himself. I mean Zama already said he's leaving in a week, so,.. AlsoI do think that the seme actually did know what he was doing. Cause when the kid said "dad" Zama thought, "at a time like this." So he really thought that the boy actually did have "those" kinds of feelings for his dad. So he left earlier so as not to get any closer and not to be in the way. They both misunderstood each other but at the same time needed the distance from each other really really get it.
I think that Taichi was alone as a child and the mayor was also lonely when he lost his wife so he took Taichi in. Taichi feels grateful to him and loves him in a fatherly way nothing more. I think Taichi has some Daddy issues but not to be romantically involved with the mayor. Taichi says he thought he could be that person's "wife" meaning that he would take the role of the late Mrs mayor so that they would stay together and not be lonely anymore and live happily. Taichi saw Zama as another father-like person since he was older and the way he acted torwards him and they met at the same special place he met his adoptive father. Zama was a trip. He's the typical seme who doesn't know what he wants and disappears without a thought. Taichi finally saw that he had a different love for Zama (ero) to the one he had for the mayor (agape) and so he was able to leave that place he felt trapped and went to live his life not being bound by that countryside and obligation to be with the mayor.
Just my 2 cents （＾_＾）