If we fight Olga it will probably be in the final chapter. She might become summonable too but if she does I have a feeling that it will be after we a Solomon servant
The Wandering Sea's doors placement looks like The Nexus from Demon's Souls. I realized way too late.
Lostbelt prologue was set in December 26th 2017 and Tunguska epilogue was in December 31th 201X (December 31th 2017 + 1 year worth of virtual time that won't be counted in the our calendar).
They usually say aloud how much time passed since the previous Lostbelt in each prologue, the only time it was left vague was the span between LB3 and LB4 with Ooku in the middle.
Adding to that, Fujimaru's recurring excuse since Heian has been "I can't verify if I'm old enough to drink or not because time is incalculable". With this elaboration that the current year is 2017+1, we can conclude that 2018 was the year Fujimaru was going to turn 20, Japan's drinking age. That sets them as born 1998. 1 year older than Mash, 6 years younger than Olga, 11 years younger than Shirou, 1 year younger than remade Shiki.
Can Shiki from 2004 beat Shirou from 2004