I would say that 想像すらしてい/ない/なかった= [here] unimaginable, unthinkable.少子化が進めば、想像すらしていなかった事態に対応しなければならないだろう。
(If the reduced birthrate goes on, we will have to cope with a situation that no one has imagined)
Past perfect use is also possible;
(Under the Versailles world order Weimar Republic suffered from an unprecedented depression)
Don't worry. I can't really tell if 想像すらしていなかった means "I haven't even imagined" or "I hadn't even imagined" or "I didn't even imagine." We just need more context. But without further context, I would translate your example as follows:The nuance in Japanese tense is really hard to understand. Although Flaminius explained a lot on it, I can't differentiate it and I still have doubt on it. I will pay attention to all of your post until I grasp it.
Yes, it is.I don't know if the original form of "想像すらしていなかった " is "想像する"?