Can I mix up plural with singular to resolve pronoun ambiguity?

In a comment Mari-Lou wrote:

....than smartphones, and for this reason they remain necessary.

And in another comment, Shoe wrote:

The theme or topic of the original sentence is computers (having larger screens than smartphones). The reason clause is a supplement. The theme of the restructured sentence is the reason why computers are still necessary (followed by the identification of the reason). So the restructuring changes more than just word order. I agree with @Mari-Lou's suggestion, which sticks more closely to the original meaning. https://en.wikipedia.org/wiki/Topic_and_comment

Technically, this is a dangling modifier. It has nothing to do with plural or singular. Dangling modifiers feel awkward because they do not attach to anything. Most people will understand you, but in a pinch, you should rewrite it.

Right now, the sentence has "Computers" as subject "larger screen" as object. "The smartphones" is modifying "the screen." "The reason" is the dangling modifier.

I think what you were trying to do is have "Computers be necessary" and everything else modifying that idea. That would be more like:

Since smartphone screens are small, Computers are still necessary.

Something like that. But not precisely sure if this is your context. Hope that helps.

I've no idea where to find supporting references, but I'd say that it is at best poor style to mix generic reference styles like this.

You could define the (complete!) which-clause more clearly:

• Computers have larger screens than smartphones, which is the reason why computers are still necessary.

But far smoother (if less punchy) is your

• The reason why computers are still necessary is that they have larger screens than smartphones.