German verbs always have to be in the second "slot" of a main clause (such as in a simple sentence).
The first slot is often occupied by the subject, but it could also be occupied by an object or an adverb, especially a time expression as here.
Do if you put "Danach" in first position, the verb "ist" has to come immediately after it in order to be in the second position of the sentence. The subject then usually comes right after the verb if it is not in front of it.
No - that would more likely be "Hinterher ist man immer schlauer" (you're always cleverer afterwards).
Also note that "danach" refers to "after that", i.e. after a specific event -- it's not completely the same as "afterwards" which can also refer to time after in general, as with your "hindsight is 20/20 afterwards" sort of context.
I think this is an edge case. You could argue that “later” should have been später in German, and it also doesn’t make reference to some event which happens first like danach does: “after that, afterwards”. But on the other hand English does often use “later” in situations like these where it’s more natural to use “danach” than “später” in German.
I’d say report it and see what the contributors think.
No - you have to translate the sentence as a whole, not word for word, because English and German often have different word order in sentences.
In this case, German has the verb in the second position, meaning that if an adverb such as danach comes first, the subject has to come after the verb, while English no longer has this rule in general speech, so Afterwards is it ... is not correct (modern) English.