Do they have to tell us or can they just say NO to whatever & go it alone? Or maybe if it's a buyout offer they have to share but with a failed or rejected partnership offer they don't?

Essentially correct, imo.  However if the buyout offer is significantly lower than the current share price they would not have to publicly share that rejected low-ball offer. Of course if an offer is anywhere near the current share price then they would have to announce it and seek & suggest shareholder approval or rejection.  JMHO