[OED] inasmuch {adverb} = [Etymology:]
originally 3 words in as much (in northern Middle English in als mikel), subsequently sometimes written as 2 words, in asmuch, and now (especially since 17th cent.) as one.†3. in as much (Without as [afterwards])
= [4.] In an equal or likedegree, likewise. (Obs[olete].)
(TL;DR) To parse inasmuch, I consider its original form as 3 separate words, per above.
1. Which do the 3 separate words (in as much) mean individually? Which definitions from OED match each word? OED is linked here for your convenience: in, as, much.
2. How did the bolded 3 morphemes combine to mean definition 4? For example, 3 contains no word that means degree in definition 4. So whence did degree originate?
Please expose and explain all hidden, missing semantic drifts. I read this, but wish to dig deeper.
Guesswork and Optional Reading (Please feel free to ignore this):
'Equate terms' in 3 and 4 above. Then:
5. as appears to equate to 'equal or like'. But this equalisation appears false, because as cannot be an adjective, while 'equal or like' is an adjective (phrase). So which definition of OED matches as?
6. much appears to equate todegree. But this is false, because much must be a determiner or pronoun, while degree is a noun. So what did much mean here?