An employment reference should confirm the dates, the role and the quality of the work, then state plainly whether you would re-hire. Check your company policy first — some employers only permit factual confirmation of dates and title.
Generally no, unless your contract or local law requires it. If you do give one, it should be accurate and fair — inaccurate references can create legal exposure in some jurisdictions.
Decline, or provide only the factual confirmation of dates and job title. A deliberately lukewarm reference is understood as a negative one by everyone reading it.
Yes. A reference without a verifiable contact carries very little weight.