Conflicting Advice: "Not Only," "But Also" Constructions -- Comma, No Comma, Parallel Structure?

I've searched for the answer on this site and other websites, and found conflicting advice and sample sentences that look wrong to me.

I'm posting this question hoping for clarification.

My understanding is that this construction can have a comma only if there are two complete sentences on both sides that follow the not only and but also bits.

The parts after not only and but also should be parallel.

But I see examples in online guides that appear wrong to me.

Wrong:

"He’s not only funny, but also he’s intelligent." (Grammarly.com)

Correct (to me):

"He's not only funny but also intelligent." (No comma)

"Not only is he funny, but also he's intelligent." (Comma)

On the same Grammarly page:

"Not only is Matthew going to Egypt for a month, but also Greece for a couple of weeks."

Why is a comma acceptable above? Why is not only not misplaced?

Correct (to me):

"Matthew is going not only to Egypt for a month but also to Greece for a couple of weeks." (No comma)

"Not only is Matthew going to Egypt for a month, but also he is going to Greece for a couple of weeks." (Comma)

Then there's the "Grammar Girl guide", which puts commas on every such sentence, example:

"He is not only a great swimmer, but also a great musician." (Grammar Girl)

Correct (to me):

"He is not only a great swimmer but also a great musician." (No comma)

"Not only is he a great swimmer, but also he is a great musician." (Comma)

Could someone please clarify the rules regarding this?

I posted what I feel is correct but I do not know where I learned the rules or whether I just made them up; and so I am not sure if I am correct here.

Fronting the sentence with 'not only A' Introduces the importance of the first element and emphasizes the unexpected element of B. When you remove 'not only' from the subject front position, then 'not only' is focusing on the combination of the two elements rather than highlighting them individually. Hence, why there's a comma when it's fronted vs. when it follows the subject.

It's correct to put a comma before "but also" whenever what ensues contains its own subject and verb because "but" is a coordinating conjunction. Whenever any of the seven coordinating conjunctions (and, or, but, for, so, yet, nor) introduces to the sentence a coordinate clause—a type of main clause—grammar requires a comma.

As for the situations when what follows "but" is not a clause, it is a matter of style. That is why the advice you're finding varies so much. Personally, I always use a comma whenever applying a parallelism that uses "not only" and "but also."

Example:

• John's not only disheveled, but also smells.

While what follows "but" isn't a main clause, it is part of a parallelism—a parallel construction that has its own special rules of grammar associated with it.

Traditionally, we have always used a comma to separate the two elements of a parallelism in a sentence, whether those two elements have been clauses or not. That, though, is changing. With the ongoing trend in grammar to eliminate commas, many style guides would no longer require a comma in the aforementioned example. When what follows the comma isn't a coordinate clause, it becomes up to you—or up to whoever wrote your given style guide, if any.

For me, where it gets dicey is when the "but" in "but also" is left implied. Here's an example where it's not implied:

• Not only is he late, but also he forgot his homework.

-or-

• He's not only late, but he also forgot his homework.

We often don't say it either of these ways, however. What we tend to do instead is leave the "but" out, so what we end up actually saying is:

• He's not only late, he also forgot his homework.

How do we properly write that, though? I really, really want to put a semicolon instead of a comma because we would normally. Not doing so makes it look like a comma splice. The rub is that it's a parallel construction, which generally requires a comma and NOT a semicolon. We don't make parallel constructions with semicolons. So what to do?