Wiktionary does say that, I see, and it surprises me. I've always-and-forever understood Inasmuch as to mean something more like "Considering that" or "Since," and Insofar as to mean "To the extent that." Collins is a little vague on the actual definition, but its usage note in lieu of definition and its example concord better with Considering that" and "Since." Maybe this bears further research—maybe even a trip upstairs to look at an old print encyclopedia. https://www.collinsdictionary.com/us/dictionary/english/inasmuch
Sorry, I found I was too lazy to go upstairs. So I went straight to God instead, since he was as near as the Kindle icon in my taskbar: He's oddly ambiguous about this as well. Inasmuch as is listed together with insofar as, yet described as "almost always inferior to because or since" — exactly the definition I've always intuited for it. I would have chosen In as much as over Inasmuch as — not that I would likely have used the phrase in a serious context — but Garner (sorry, did I misspell that as God a moment ago?—it's an easy mistake to make) informs us the preferred renderings are Inasmuch as and In so far as.
So here are my interpretations. My Inasmuch as is supported by about half the dictionary definitions in the first page of Google results — and equally, contradicted by the other half. Inasmuch as [because] you never returned your countersigned acceptance, you're not a member in good standing. Insofar [to the extent that] we've been able to determine, you're not descended from Thomas Jefferson. Half the dictionaries in the quick-search results instead consider Inasmuch as and Insofar as both to mean to the extent/degree that. Presumably they would bless: ?? Inasmuch as [to the extent that] we've been able to determine, you're not descended from Thomas Jefferson. [just does not sit well in my ear] Garner implies they're synonyms — but then more strongly imputes to Inasmuch as the definition because or since. Would he give the same interpretation to Insofar as? Another riddle to lose sleep over.
I'm glad I'm not the only one who frets over these things. You're probably now a leading expert on the derivation. You are almost compelled to put it into a story. haha
I couldn't rest until I checked it in the OED, which I've just now done. Not because I want to be "right" — in fact, the risk of turning such questions into competitions is a constant deterrent. But as you appear to recognize from yourself, I simply had to get to the bottom of it. Plus few things make better procrastination from actually writing, of course. Both usages go back to the 1300s. Collins lists because/since first, and OED lists it second. Additionally, OED reminds us of a usage without as: From OED There it's equivalent to the correct modern usage of as such. And let's not get started on the ways that little phrase gets abused.
Just observed this in Dostoyevsky's Demons. One caveat: I am reading it in Russian. He uses full sentence asides within those em-dashes like it's nobody's business, creating that multi-layered polyphonic structure of his. Ridiculous skill.
Start off by knowing that just because someone is a published author doesn't mean they do everything by the book. In this case I think it was not a good call onthe writer's part. Otherwise, one might get creative with punctuation and font, if the content supports it. As it stands, it's not to Hoyle and it's not even helpful because when I read it I felt dialogue, like someone was shouting, and yet that doesn't make sense in context.
