@jkerian ここをどう考えるか is a noun clause (roughly “how to handle this”), and therefore ここをどう考えるかです is the usual [noun]+です. The subject of …です is omitted because it is clear from context; think of, say, 重要なのは as the omitted subject.
"のは" I think here means "the reason that", so I think everything after it is in the quote, i.e. "That time, the reason that the greatest reality came from beyond to meet was to come and laugh 'our existence is this simplistic'"
the translation "Back then, the greatest reality came to us from beyond purely to laugh at the primitivity of our existence" on that page I think might be pretty close
@jkerian Your translation is fine, although it is probably better to translate ここ as “this (point)” rather than “here.” I am not sure what you mean by “related to the んです construction.” Well, it is related because both are [noun]+です, but nominalizer ん is not used in ここをどう考えるかです because ここをどう考えるか is a noun clause from the beginning.