Why aren't Len and Satsuki part of the volleyball game in episode 2? Because they weren't playable in the original Melty Blood!
Also the volleyball game took place during the day and Satsuki is a Dead Apostle so she'd probably be asleep or too weakened to play.
That's a possibility, but that didn't stop Sion from participating. Satsuki most likely simply wasn't invited.
In the dancing part in the opening, why is Archer immediately after Shirou, when the previous lines established it as "Tsukihime character followed by a Fate Stay Night character" and so on and so forth? Then you remember: "Oh yeah, Shirou and Archer are the same person."
But, then why are Lancer and Kirei right next to each other?
There aren't enough male characters in Tsukihime to keep the pattern going. The highest potential number is 4: Shiki, Nrvnqsr, Roa/SHIKI, and Arihiko. Kouma and Wallachia are possibilities, but they're in the background.
In the "Magicians Hate Machines" short, Archer is enjoying the hell out of Rin's freakout. After noticing the clock, he thinks to himself "it's almost time" and finally explains to her what she needed to know, without really having been told what it was Shirou was wanting recorded in the first place. Then you realize again that Archer is Shirou from the future and doesn't like his past self very much, so of course he wouldn't miss out on a chance to jerk him around!
During Season 2, the Neko Arcs are concerned about the cafe being in the red. But, by the time Episode 8 ends, they have Saber and Gilgamesh to thank for putting them back into the black.
It doesn't last as the cafe is back in the red by episode 10.
Caster's been learning Soichirou's martial arts from him after being on the wrong side of an I Surrender, Suckers moment from Rin back in the original novel. This explains why she's able to deliver a knockout strike to Issei in that style in episode 7.