Learn Dutch: Kortom, de schrijver zou de plot veranderen. - In short, the writer would change the plot.

While adverbs at the start of a sentence normally push the verb before the subject, kortom is often treated as an introductory remark outside the main clause. Because it is set off by a comma here, the main clause just uses normal Subject-Verb order: de schrijver zou.

Is kortom related to the word kort?

Yes, it combines kort (short) and om (around). You can think of it as bringing a story 'shortly around' to its conclusion, which perfectly matches its meaning: 'in short' or 'to sum up'.

Why use zou veranderen instead of the simple past veranderde?

Zou veranderen means 'would change', which makes it a conditional or 'future in the past' statement. It shows what the writer intended, hypothesized, or was expected to do, whereas veranderde (changed) would mean the action definitely already happened as a fact.

I see de plot is an English loanword. Are most English loanwords in Dutch de-words?

Yes! When English nouns are adopted into Dutch, they almost always take the de article. So saying het plot is incorrect and would sound unnatural to a native speaker.