Not sure if this is what you're looking for. I didn't read all of the post here. Anyway, if you're wanting to open photos in full screen that you've opened in the finder, without adding them in to iphoto:
1. Find the file with the photos you're wanting to open.
a. ie. a folder called photos on your desktop.
b. Click on the folder to open it.
2. Once this folder is open.
A. select the photos you want by holding your left mouse button down, drag over the files to highlight the
ones you want to open or.
B. press the command key and the "a" at the same time to highlight all of the files, or
C. There are several more ways to do this, that you propably know...........
3. Then, right click and press "open" or double check on one of the photos.
At that point, preview should open. If you have your view set to thumbnails, you'll see all of the photos you selected along the left side of the screen and a larger view to the right. You can scroll from photo to photo using 2 fingers(swiping up or down). Or right to left on the apple mouse. If you want to go full screen, click on the double arrows at the top right hand side of the window and move from photo to photo the same way.
If the folder you opened initially contains other folders with more photos in it, and you highlighted these folders as well as individual jpgs,, preview will open these folders in a separate window. Then repeat the process above.
Hope this helps.