I have been a mac user for 5 years and in that time, I have bought 5 macs. I just love them.
As for undoing photo edits, you have a few options. The first is to just go to the menu at the top, and click on edit, and keep clicking undo until you get back to the original photo.
The next option is to click on photo, and on the drop down menu at the bottom there is an option called revert to original. This is the most efficient.
Also, when editing pictures in the adjust pane, there is reset button at the bottom of the window. I have only used this when I'm editing the picture so I'm not sure if it works once you close the window.
One thing you should understand is that when using iPhoto, it does not destroy the original photo. It actually keeps it. This is good and bad. Good because you can always revert to the original photo. The bad is that it takes up more memory.
As for using the mac, I would suggest just using it, and you will get used to it. If you need some help, the Missing Manual series by David Pogue is awesome. He is a great writer for the N.Y. Times, and his Missing Manual series is very helpful. They sell a Missing Manual for various topics. They have one for Leopard and another for iLife. I just don't know if they are for the most recent versions. But I'd totally recommend them.
Also, the Apple support forums on their support page is good. Lastly, if you navigate around the Apple Support website, there are lots of videos on how to do things.
Good luck.