tale otherwise so utterly improbable

Solution 1:

No, it would not be rewritten as “already.” Think of it as “even without which it would be considered . . .”

It is not a different use of the word from that in use today. Mary Shelley is an adroit user of language, and her text does not seem out-of-date today.

You yourself report searching the 19th century books in Google Ngram and not finding any use of “otherwise” meaning “already.”

Here’s the full paragraph:

Day dawned; and I directed my steps towards the town. The gates were open; and I hastened to my father's house. My first thought was to discover what I knew of the murderer, and cause instant pursuit to be made. But I paused when I reflected on the story that I had to tell. A being whom I myself had formed, and endued with life, had met me at midnight among the precipices of an inaccessible mountain. I remembered also the nervous fever with which I had been seized just at the time that I dated my creation, and which would give an air of delirium to a tale otherwise so utterly improbable. I well knew that if any other had communicated such a relation to me, I should have looked upon it as the ravings of insanity. Besides, the strange nature of the animal would elude all pursuit, even if I were so far credited as to persuade my relatives to commence it. Besides, of what use would be pursuit? Who could arrest a creature capable of scaling the overhanging sides of Mont Salêve? These reflections determined me, and I resolved to remain silent.

Solution 2:

The quoted text uses otherwise as an adjective to qualify tale. It might also be argued that otherwise is used as an adverb to qualify the implied elliptical verb in “… tale {that is} otherwise so…}”. Let us consider both possibilities:

Otherwise adjective = used to show that something is completely different from what you think it is or from what was previously stated

Otherwise adverb = differently, or in another way

Cambridge Dictionary

The same meanings are expressed slightly differently in

Merriam Webster

The manners or timings of the differences between the two compared things are not explicit in these meanings of otherwise. Neither interpretation is consistent with the notion that otherwise may be replaced in contemporary usage by already.

Now consider already. Merriam Webster seems to echo the generally held view that this is an adverb:

Already adverb = prior to a specified or implied past, present, or future time : by this time

Merriam Webster

Also see relevant discussion of already in:

Crown Academy

And so it is that we have to consider otherwise (adjective and adverb) and already (adverb). The question assumes an equivalence between otherwise and already that must therefore only apply to the adverbial uses.

Shelley writes “… a tale otherwise so utterly improbable …”, which may be reasonably taken adverbially to refer to “… a tale {that is, even when seen differently} so utterly improbable …”. This is consistent with the contemporary usage discussed above.