I think there is a major error in the first 3a, unless I am very much mistaken( which is always possible). I was sure that one may not be with his wife on Shabbos because it’s really Tishah B’av and therefore D’varim Sheb’tzina would be Noheg.
Also, about the whole learning on Erev Tishah B’av thing. It’s a Machlokes Haposkim and not such a prevalent Minhag. I’m not sure why the author treated it almost as a given Halachah for everyone. Therefore, his reason for not having meat at a Siyum after Chatzos shouldn’t be relevant to the majority of people. However, one should never make a Siyum on a Friday afternoon after Chatzos because one shouldn’t have a festive meal on Friday afternoons that one wouldn’t normally have as it’s considered a lack of Kavod Shabbos.