The seme was at the marriage meeting because he worked there but was undercover because there had been a lot of frauds so his boss probably sent him in to watch over at a closer view. I'm not sure about the second question but for the third question, seme was looking into ukes profile that he probably sent in to get the marriage meeting membership thing or whatever. I am also not so sure about the last 2 questions but I hoped that helped your questions even if it's just a bit.
adding to what the person above me said, i believe he may have had to fill out an interest form (ie: what he liked and disliked, in order to attend the party/gathering.) he may have been observing the uke because he was interested in him, and noticed that his body language/ conversations with women were all stiff or forced. as to where the young uke found the book of homosexual content, it is not explicitly stated in the manga, so it’s up to you to interpret. my guess is that the uke found it from a neighbor/older kid in the neighborhood. sorry this is a lot, hope it helped if you read all of this.
She don't have gay books .. it was a bookstore where uke found that book and wanted to read it and his mom is homophobic that's why she said not to read that book
Wait im so confused. The seme in the first story was so weird. Why was he at a marriage meeting in the first place if he knew he was gay? How did he already know so much about Iwaizumi when planning the date? How did he know that Iwaizumi was going to another marriage council? How did he know Iwaizumi didn't like women? What's up with Iwaizumi's mom if she was homophobic why did she have gay books lying around???????????????