unknown faces on photos

It will depend on your version of Photos and MacOS X, if you can remove an unknown face or not. In Photos 3.0 on or 4.0 you can only remove faces you added yourself. The newer version of Photos are always keeping all automatically detected faces. You can only disable the display of the faces circles, so they will no longer be shown as overlays on the photos, when viewing the photos enlarged (View > Show/Hide Faces Names)

To remove a faces circle view the photos enlarged. If there is a "x" on the circle, click it to remove it.

• In Photos 1 and 2 you can remove all faces circles.
• In Photos 3 and 4 you can only remove the faces you added yourself.

"Also, when I click on a photo a different photo pops up and I cannot retrieve the photo I am seeing originally"

This looks like a problem with the thumbnails in your library. Some thumbnails may be bound to different photos. If the thumbnails are wrong, you can create new thumbnails for selected photos this way, see: How to Rebuild Thumbnails in a Photos Library

• Select all photos that need new thumbnails at once - it does not matter if the selection includes photos that do not need repairing.
• Then rotate the selected photos back and forth by first pressing ⌘R (Image > Rotate Counterclockwise) and then back by ⇧⌘R (Image > Rotate Clockwise).