Both should be accepted here; they weren't before but I've added that now. The general idea is that det is often it but can also be that (in most cases, both should be accepted answers in English).
that can be det or det där (depending on the Swedish sentence: in many cases only one of them works in Swedish)
det där is always that
this is det här or detta and vice versa.
I disagree. Considering that "In any case" might be a useful, I'd rather keep it. Indeed, the sentence itself is stupid, but in my opinion duolingo is not for learning useful sentences. It teaches useful language concepts, what you can expand further. You might learn a bunch of useful sentences, but without stupid useless phrases it is hard to facilitate understanding the language rather than learning the "dictionary of duolingo example cases"