plural or singular in this sentence

Question

1. The product and the scale have changed from a small prototype to many production units.

2. The product along with the scale has changed from a small prototype to many production units.

Can sentence-1 be seen as incorrect, perhaps semantically, because it uses plural "have" but applies to "a" small prototype?

The issue that the plural "the product and the scale" is being applied to a singular predicate noun of "a small prototype." While the subject (product and scale) is plural and agrees with the verb (have), it does not agree with the predicate noun (a small prototype).

Answer 1

because it uses plural "have" but applies to "a" small prototype?

No, because it doesn't, it applies to the subject "the product and the scale", and that's plural.

Answer 2

The problem, as Edwin Ashworth alluded to, is not a question of plural vs.singular; it is that you have lumped "product" and "scale" together in a way that invites confusion. What you are trying to say is that the product has changed such that it is no longer a prototype, AND the scale has changed from one to many. You need to keep these two concepts separate enough to make them understandable. You can't just throw them both into a sentence and expect the reader to sort out which change applies to which of the subjects.

Try something like this: "The nature and scale of production have changed, from crafting a single protype to manufacturing many production units."

Answer 3

A singular verb certainly can be appropriate with linked nouns that superficially appear to create a plural subject. For example, in a discussion of cocktails at a bar:

The gin and tonic is excellent.

But the sentence

The product and the scale has changed from a small prototype to many production units.

doesn't present the reader with a similarly unitary combination. This would be clearer if you identified the nature of the vague word scale in the sentence. My sense is that scale stands for something like "scale of production," in which case the complete sentence would read like this:

The product and the scale of production has changed from a small prototype to many production units.

The dissimilarity of the ideas "product" and "scale of production" makes the attempt to treat them as a unified, singular subject ill advised, in my opinion. Suppose that the product under discussion were Cadbury's new Whizzie Bar, and that after fiddling with a test prototype of the candy, Cadbury had settled on a recipe and decided to mass-produce the bar. That would give us this sentence:

The Whizzie Bar and the scale of production has changed from a small prototype to many production units.

But to my mind, the plural-verb alternative is clearly preferable:

The Whizzie Bar and its scale of production have changed from a small prototype to many production units.

The more concrete version of the sentence also reveals a deeper problem with the sentence. If we break out the two assertions embedded in it, we get these two statements:

The Whizzie Bar has changed from a small prototype to many production units.

The Whizzie Bar's scale of production has changed from a small prototype to many production units.

Examined separately, neither of these assertions is especially well expressed. The "scale of production" assertion would make more sense if we replaced small with single:

The Whizzie Bar's scale of production has changed from a single prototype to many production units.

since "small" in scale implies something different from "a small prototype," and "a single prototype" makes clear what the scale of production was at the outset.

Bu the bigger discovery involves the "Whizzie Bar has changed" assertion. To argue that the Whizzie Bar itself has "changed ... to many production units" is to commit an error against sense—because, once the prototype Whizzie Bar is adopted, it doesn't change. The product remains the same whether it is produced on a scale of one bar per week or 15,000 bars per week. Therefore, going back to your original sentence, the embedded assertion

The product has changed to many production units.

simply doesn't make sense. The product hasn't changed; the scale of production has changed. It's hard to think of clearer evidence that the original sentence combines two nonunitary subjects and therefore ought to take a plural verb. Further, since the subjects "product" and "scale of production" don't apply to the same objects whether the verb is singular or plural, they really shouldn't be put to the attempt. I recommend revamping the sentence to something like this:

The product, which began as a single prototype, is now in large-scale production.