MarkD -
"Is Air Traffic Control obliged to continue to offer clearances where previous offers have been declined?"
Lets turn it around a bit... lets say the aircraft had a mechanical whilst lined up in queue for departure, and declined clearance to return to gate/ramp for maintenance. Would the controller hesitate/be obliged to provide a second clearance in that instance? I would think that severe weather is just as much a hazard as a mechanical.
That said there are some airports (such as LAX) where departures and arrivals are keenly rationed, and require a departure slot. In these situations a second clearance would be dependent on coordinating a new (time) window.
J.