I'd like to update one of the old theories here and also add a new one, although the "new" one isn't entirely new.
Starting with the update, I want to reorder the "original" 1st string roster slightly. This just concerns the G10 though, so I won't bother listing the rest again:
Putting that into a standard line-up:
Reasoning: we know that Irie was supposed to be part of the 1st string before Tokugawa joined the camp. So what number is the most likely for him? Oni was No. 5 before he gave up his spot and he became the 5th court guardian afterwards. Tokugawa was aiming for the No. 1 spot and joined 1st court. Finally, Irie was part of 3rd court, so chances are his spot was supposed to be No. 3.
1st Match: Tohno/Ochi
2nd Match: Kimijima
3rd Match: Kaji/Ohmagari
4th Match: Oni
5th Match: Irie/Tanegashima
6th Match: Watanabe
7th Match: Byoudouin
This isn't anything too new and I'm sure I've talked about this with several people before. The real update here is to put Duke as No. 2 and Tanegashima as No. 4 as opposed to the other way around. But I think it makes sense that way for two reasons.
First, I still think that Irie and Tanegashima were a doubles pair and going by the pattern that's 3/4 and not 2/3. However, second and more importantly, Tanegashima is supposed to be equal to Oni, so wouldn't it make a lot more sense for them to be right next to each other on the line-up as opposed to three spots apart? So this looks pretty good to me.
Of course it also means Tanegashima improved enough to overtake Duke over the course of the year.
Now for the somewhat new theory. Those that have been around for a while may be familiar with my obsession over how the 9/10 challenge match developed, and also that I believe Ochi stopped using Mach against Niou during the tiebreak. We know Niou can't return Mach and that Ochi's serves never miss, yet the 2nd string must have scored when Ochi was serving and Niou was receiving here, or else they wouldn't have been able to make up a 6 point deficit. Now, the big question is why would Ochi do that?
I think the answer lies in the Q&A from the fanbook:
I think in this particular part, Konomi means that to Ochi the racket is like a weapon, so he takes good care of it. But if he imagined him as a warrior, that probably extends beyond racket maintenance. So what if Ochi stopped acing Niou because Niou became unable to hit ZSS? If it's a matter of honor or pride for him, similarly to how he allowed Kabaji's point, I could definitely see that and it would clear up one mystery.