Will answer as best I can in the same order as your questions.
1. You can use it immediately.
2. Yes it does allow for both nose and mouth breathing.
3. Fillings are fine but the crowns may be impacted by pressure from the mouthpiece. These MAD type mouthpieces are generally not recommended for people with dentures or extreme dental work like bridges, crowns, caps etc. I do know people who have still used them and not had in adverse effects but it will be a risk you will be taking if you do try them.
4. Yes – should still be okay.
5. Yes – should be okay
The author left out a step during the final editing portion on Photoshop or Gimp. After you invert the colors, you should see that everything is blue. You need to select white balance and click on a part of the image you know should be white, then go to black balance, and click on part of the image you know should be black. Then everything will turn to color. You may see that there is a lot of yellow spots as well, because instead of evenly distributing the light, the lamp just causes one big overpowering bright spot. I've tried so many things, but nothing gives very good quality other than a scanner that really has the capability for scanning transparencies and negatives, unfortunately.