The original manuscript is not extant here. In the printer’s manuscript, Oliver Cowdery initially wrote “& prepared the way whereby”. After writing whereby, Oliver erased the wh and overwrote it with th (the ink flow is somewhat heavier). At the same time, he made two further corrections: he changed the definite article the to the indefinite a (by crossing out the the and supralinearly writing in the a), and he supralinearly inserted the connector that that he had accidentally omitted (like the overwriting for thereby, these two corrections are written with somewhat heavier ink flow). Here Oliver’s error may have been influenced by language earlier in the text:
As far as the article for way is concerned, either article is theoretically possible. Elsewhere in the text, there are 6 instances of “prepare a way” and 12 of “prepare the way”. So there would have been little motivation for Oliver to emend the to a in this verse; 𝓞 likely read a way rather than the way. Turning to thereby, we note that this word needs a preceding that while whereby does not: everywhere else in the text when thereby begins a subordinate clause (35 times), there is always a preceding that; on the other hand, the relative pronoun whereby is never preceded by that (out of 24 occurrences). Thus it seems reasonable to assume that 𝓞 read that thereby in Ether 12:8.
Summary: Maintain in Ether 12:8 the corrected reading in 𝓟: “and prepared a way that thereby others might be partakers of the heavenly gift”.