Learn Italian: Innanzitutto è tardi; inoltre sono stanco. - First of all it is late; besides, I am tired.

Yes, they can be used interchangeably here, but they carry slightly different nuances. inoltre is a standard, relatively neutral way to say furthermore or besides. per di più and oltretutto are more emphatic, highlighting that being tired is an extra, perhaps heavier burden on top of it already being late.

How do innanzitutto and inoltre work together in this argument?

They act as structural signposts. innanzitutto (first of all) introduces the primary or most urgent reason for stopping or leaving, which is è tardi. Then, inoltre introduces the secondary, supporting reason. Together, they make your excuse sound highly organized and persuasive.

Why is a semicolon used before inoltre rather than a comma?

In Italian writing, a semicolon is strongly preferred over a comma when joining two independent clauses, especially before connective adverbs like inoltre, tuttavia, or perciò. Using a comma here would be considered a punctuation error in formal writing, while a full period might disrupt the flow of these two closely linked reasons.