First off get a file manager. Such as Astro file so you can manage files to your network Laptop Desktop ect. https://play.google.com/store/apps/details?id=com.metago.astro
Your not going to see the raw files in Android natively. You should probably learn more about file formats for Photography.
Look at the Dcim area for the raw files marked DCM extension on your phone.
This is a Raw file viewer that allows you to share to Snapseed https://play.google.com/store/apps/details?id=com.BrainyLantern.slingShotPortfolio
Look at the top right for open in editor after selecting a photo. DMG extension
Make sure you enable the Ap permissions otherwise you won't see your files when installing the apphttp://digital-photography-school.com/raw-vs-jpeg/
If you payed for Snapseed you were ripped off its a free application.
Far as Manual camera. Just minimize then
swipe left to exit.
Camera FV-5 Payed is a way better app to use for Raw shooting. I have the same phone BTW but I still sometimes import from my DSLR to Snapseed.
Most of the time I use Adobe Light room it's quicker and I can batch edit photos.
Check out Photo mate R3 a way better way to edit with tons of features works best on a tablet.
Words of wisdom don't post camera questions in the Nexus Community all you get is amateur answers.
You can look at my profile on experience with Photography.