I think you're dead on with the timetable. If the slots are awarded by Dec of 2006, the entire $290M will have to be made available no later than March of 2007. That would require them to renew the Mellon Arena lease for at least one more year (the 2007-08 season) and possibly two (they'd have to add another option year for the 08-09 season, but with the funding guaranteed and the construction ongoing, that wouldn't be much of a problem at all). So at the VERY latest, the new building would open in the 2009-10 season.
Second, with that 90-day availability clause written into their contract, they could theoretically break ground on the new arena the day after the license is awarded.