Not sure how Ooku is any less of a Main Interlude chapter than SERAPH.
Not sure how Ooku is any less of a Main Interlude chapter than SERAPH.
What is Main Story relevance in Ooku?
IIRC it is just Parvati saying that something is wrong in LB4 with gods and that she wouldn't help there, but LB4 would have another helper.
Seraph had a pillar demon.
And Scramble was labelled as 4.5, while I am not remember Ooku being called that.
A Beast, and a direct connection with LB4 as well as hints at its situation is more than enough story relevance when you consider that it's a story event, and not a full chapter. The pillars don't actually matter in the grand scheme of things because they could've just as well all disappeared with Goetia, so Seraph's relevance is another Beast, so in that sense it has arguably less relevance than Ooku following this logic. Scramble, on the other hand, has none of this. Its story is effectively (for now) entirely isolated (though I'm still waiting for that thing Gogh says at the end about protection from the "other side" to have some relevance), and its relevance as a "story chapter" comes entirely from its explicit positioning between LB4 and LB5 (because it deals with the test run of the Nautilus, which in LB5 was only referenced in a single throwaway line as "this happened between then and now"). Heian as 5.5 and Tunguska as "6.4" (for lack of better numbering) have more direct relevance as story events/main interludes in the sense that they are the climax of character arcs for characters that have been present for most of the main story up to this point, but when you think about it, both of those also happen to deal with Beasts. We still don't know what 6.5 will be about, but at the end of the day all but one of the .5 chapters has actually dealt with Beasts specifically, because this just happens to be one of the biggest subplots in FGO.
As for when Ooku was numbered, it was in the Famitsu 5th anni Q&A interview thing (at least going by context from what's posted around this in my discord server):
Nasu: Experimentally, we have Chapters 3.5 and 4.5 in Part 2. The Tokugawa Labyrinth Ooku was Chapter 3.5, and so similarly there is a Chapter 4.5 which will be released in the latter half of 2020. In that way, big stories will be released between chapters after Chapter 5, so including those, in terms of volume Part 2 is only halfway done.