Our 3rd side-scene went very well. 1 player had to drop out just before the climactic sacrifice!
Three guardian types that work/breathe in the arcane library vs. big bad epic level wizard intruder arrived to take out the mage council. The 3 tPCs tried their best, but impossible odds were stacked against the defenders. Though no life lost, the consequences of the character’s interference will be felt for sessions to come.
Next session, I estimate the main PCs will arrive at the scene just after the 3rd side-scene explosion, but I’ve been wrong in my estimates before. My players have been pretty good about not metagaming information between their alternate characters.