If you have the 3.5l V6 I'll be willing to bet it's the low pressure hose. I read that this is pretty common for the 2002 to 2005. Mine when out on me at 60,000 miles. The cost for a new hose $100 bucks from the Nissan dealership. Not including installation, and freeon recharging.
Depends, you may still have a leak that the technicians did not find the first time or maybe your AC compressor isn't working now. If you still hear the AC compressor cycling then most likely it's a leak.