And you also don't know what Soske saw. He never said that he had seen a sex video, he only said that he had evidence of adultery. But what Soske means by the words "infidelity" we can only guess. It can be both sex or a passionate kiss with preliminary caresses. Therefore, it's wrong to claim that he saw a sex video. I'll ask you to indicate the chapter where he says he's seen a sex video.
It doesn't matter how many months have passed since the publication of that chapter, the only thing that matters is what Shingo thought then. In the previous comments I already wrote why we can't trust Shingo's testimony.
Your entire conjecture is based on assumptions, out of pseudo blank points. The manga not being explicit about shown contents does not mean they don't exist. It isn't even diabolical proof at this point, as we saw Rika and Shingo kissing, and Sousuke ackowledged the adultery in the video. Trying to pin this on subjectivism, at this point, is shallow deflection, and you know it. Or what, you wanted a special hentai chapter of unmasked Rika making out with Shingo as definitive proof?
Again, the "Shingo's testimony can't be trusted" has been debunked. He wanted Rika to fuel his SNS addiction( requiring him, Shingo, to be on camera). On his monologue, he addressed the sex. He broke when exposed on SNS. On his offscreen testimony, past his broken point, he acknowledged the sex with Rika using protection( chapter 73). He was backstabbed by her with the SD card contents deleted. You're mixing two different momentums( Shingo pre-break and post-break) to draw one personality and deliver a point. It doesn't work, as much we try. His testimony came after break, so it's valid.
Lastly, the 10 months DO matter: because it gave us new characters, arcs, and story. And you still persist on Shingo's testimony being flunked, worked on Misato being more of an agent to break Sousuke's marriage, still persist in Akiba being dangerous to Sousuke's marriage...but funny, you barely address Iwabuchi as a negative point nowadays, not since Rika made sure, in all letters, to say he's an ally of her.