Oliver Cowdery’s insertion of fruit in the printer’s manuscript seems to be a virtually immediate correction to the original manuscript (which is no longer extant here). The level of ink flow is the same. And much is never otherwise used as the head of a noun phrase in the olive tree allegory; for noun phrases, we find much used only as the determiner for a head noun, such as “much strength” (three times) and “much (evil) fruit” (ten times). Moreover, in the allegory there are 29 clauses that refer to “bringing forth fruit”, including this one in Jacob 5:20.
Summary: Retain “much fruit” in Jacob 5:20, Oliver Cowdery’s correction in 𝓟; the original manuscript, no longer extant here, undoubtedly read this way.