It is great that you looked up so many proposed translations!
The many routes taken reflect the difficulty of translating well and the necessity to choose goals for the translation.
Google Translate is unreliable with Latin; for detailed analysis and mockery, see the linked question.
The original quote is a line from a poem written in dactylic hexameter.
The last syllable in proinde is short but the first one is not split in two.
Correcting this makes the rest of the line scan naturally:
Proindĕ, lĭ/cet quam/vīs ex / ūnō/quōquĕ lŏ/cō sōl
The caesura seems to take place between quamvis and ex.
I do not find a similarly suitable place for it ...
I think your last suggestion is correct: "The ocean is very big (such a space!), so even if only a little water is lost in any one spot, a vast amount is lost over the ocean as a whole".
The other options do not make nearly as much sense to me.
Notice also that proinde ("therefore, so") refers to a preceding statement, here the previous line.
You could ...
You are correct that the a.c.i. (and some other constructions) is called indirect speech. So Lucretius could have written, in a new sentence, nequaquam divinitus est creata natura, which would not have been indirect speech. But I think the main point here is that indirect speech is not necessarily speech from someone other than the speaker: it can be the ...