Alright, decision time!
Looking at the first scenario, 妊娠カレンダー is now 3 votes ahead of サイレント・ウィッチ, so that seems pretty clear to me.
Regarding who would read along with the respective club, we get 21 votes for 妊娠カレンダー and 20 votes for サイレント・ウィッチ, which is also a very good result.
Looking at the second scenario, it might look like reading in parallel is also a viable option, but if we count the votes, it turns out that 12 + 3 + 4 = 19 people would read 妊娠カレンダー while 7 + 4 + 3 = 14 people would read サイレント・ウィッチ.
So the former would not lose many readers but the latter would go in with a much smaller group, and given the fact that the book club host puts in quite some effort to start with, plus it’s much more fun to read in a larger group with more discussions and whatnot, plus there are always dropouts for all sorts of reasons, I think reading in parallel is not a good idea for this constellation.
Now for the extra scenario: Thank you to everybody who raised their concerns. I did not comment on them yet, but I must confess I was not a big fan of this option from the start, and I just included it because we had such a poll already in the past, and so I thought that maybe we can learn something interesting from it.
Although many people agreed with reading the books back to back without a poll, given the large number of “disagree” and “not sure” votes, I don’t think this is a good solution either. If サイレント・ウィッチ is still so popular in the next voting round, I think it will easily win, and so we get to the same result while at the same time having a fresh decision, which is more interesting as well. (Also, we should not forget that this is the IBC so we can expect quite some fluctuation as new people join the club and others move on to higher levels.)
TL;DR:
Our Winner is: 妊娠カレンダー

and we will have a regular poll once it’s finished.
Thank you to everybody who participated in the polls, the discussions and the analysis, I did not reply to your comments but I read them all, and you all helped me a lot with getting to this conclusion.
Also, my apologies to @Redglare for the discussion-gone-wrong
I should have spoken up earlier but I did not realize that it could be hurtful for you and everybody who really likes the book. I hope that our next poll will take us back to our former discussion standard 
Now back to the book club business: @Domjcw would you like to run the club for 妊娠カレンダー ? It is scheduled to start on October 19th.