Measure how much text needs paraphrasing.
A similarity score is matched words over total words, but the number alone means little. What matters is whether the matched text is properly quoted and cited: a fifteen percent score consisting entirely of correctly attributed quotations is fine, while an eight percent score of unquoted borrowed phrasing is not. Subtracting quoted words from matched words isolates the portion that genuinely needs rewriting.
Similarity
Similarity = matched words / total words x 100; quotation share = quoted words / total words x 100
Rewriting required
Words to paraphrase = matched words - total words x acceptable threshold / 100
No. A very low score alongside few citations can indicate under-referencing. The score is a flag for human review, not a verdict on its own.
Only with a citation. Rewording someone's idea without attributing it is still plagiarism, however different the wording.