Translation:You do not need to come if you do not want to.
This is a word-for-word translation which is perfectly good English, indeed, the contracted form 'You needn't come if ...' is arguably as prevalent as the 'You do not need to come ...' or 'You don't need to come ...'.
Is there a specific rule/reason why it is "om du inte vill" and not "om du vill inte"?
The adverb comes before the verb in subordinate clauses. So it’s du vill inte but om du inte vill.
I would add as acceptable "You don't need to come if you don't want." The final "to" is often omitted in colloquial English.
I know it's not uncommon colloquially, but it would likely still be considered ungrammatical in text, which makes me hesitant to add it - since a lot of our users also take the course to learn English. I do see your point, though. Will give it some thought.