PUT PHOTOS ALBUM IN SIDEBAR IN FINDER

The sidebar can only hold folders and volumes, but your Photos Library is a bundled package and not a plain folder. SO you cannot drag it to the sidebar.

But you could add it to the toolbar of every Finder window , like this:

To add your Photos Library to the Toolbar, open two Finder windows showing your Pictures folder.

Then go to the main menu bar and use the command "View > Customize Toolbar".

Now drag your Photos Library from the second Finder Window to the toolbar in the window with the Toolbar options.

Select "Icon Only" and click done. You can now open your Photos Library by clicking the icon in any open Finder window,

Also - what would happen if I deleted my photos from the pictures folder - would they still be in the photos folder?

As to your second question - that depends on how you imported the photos to the Photos Library. Photos has two ways of importing: managed or referenced.

1. Managed Library: If you are importing photos with the default option "Photos > Preferences > General> Importing: Copy items to the Photos Library" enabled, Photos will copy any photo into the library and you can delete the photos you already imported to Photos from their original location.
2. Referenced Library: If you are importing photos with the default option "Photos > Preferences > General> Importing: Copy items to the Photos Library" turned off, Photos will not make its own copies and reference the photos in their original location. Then you must not delete the photos after importing them., or you will lose the photos.

A referenced library is more difficult to manage because you are responsible to protect the originals and it is easy to make mistakes. And Photos does not have tools to recover from such mistakes.

So, how did you import your photos, managed or referenced?