Nuvi 350 questions

Welcome aboard. I'm sure you will enjoy this site. Warning, this site can be addictive.
To your particular questions:
1. You can't update or change the built in POI's. Garmin does that on a yearly basis with new maps ($75). Garmin depends upon Navteq for the data. You can report errors or changes to Navteq (http://mapreporter.navteq.com/dur-web-external/) and hopefully they will be incorporated in the next set of maps.
2. You can load .cvs and .gpx POI files using a program called POI Loader from Garmin (see FAQs). One of the reasons people create these files is to keep an up to date listing i.e. have the correct store locations for Wal-Mart. There are many other reasons to use these custom POIs. Read the FAQ's and getting started.
3. Many users have reported good results with the fm traffic service. It depends on where you live. You can do a search on this site for Traffic to see the many responses.

You can't delete them. You're stuck with 'em 'till a new version of the map comes out, and hopefully, they've removed the stale info. If you want to make sure they have the info about things that do exist, and are not on the map, or things that are there but don't exist any more, go to NavTeq's Map Reporter and fill out the form for the issue.

And yes, you can load csv files or gpx files with the POI loader program - at the top of this page, click the "getting started" link.

Finally, FM Traffic: the service is only as good as the radio station suppling the information, and the signal in the covered area. If you're out here in the middle of flyover country as I am, the traffic thing is worthless. Best you can do is try it out, and if you like it, sign up when the free trial is over.