Of course the heat shield is there for a purpose.
Mine was making noise and I just attached a plastic clip to stop the rattling. If your able bodied, it’s good to peek under there instead of Nissan charging you to fix something.
Mine is not corroded after 10 years because it don’t snow much.
I usually touch up rust on the underbody with rust convertor and a paintbrush.
Used a spray can version sometimes.
Had to spray the axles arms since those were rusty since my dumb dealership wanted to just replace the boots instead of a new replacement. Runs great but the repair cost would have been close to a new axle.
Good luck with your dealer.