grammar - 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.