Hi ,
First of all, don't be afraid to ask questions if you are still confused after reading. It's the people who don't bother reading that waste everyone's time on these forums.
1. Yes these instructions are still applicable to any new s3 from at&t if you use a PC (Odin is PC only). Rooting on a Mac is a slightly different process indeed and one I plan to add to the OP, but you should find info on how with a little searching. Otherwise, you just need a PC for the initial flash, borrow one, or run Windows on your Mac using boot camp.
2. Don't worry about the flash counter as there is an app which can reset it for you: chainfire's triangle away app (look for it in the forums)
3. You can't backup without root access unfortunately. However, you can easily return to a stock phone by flashing the original image through Odin or mobile Odin. All you do is grab the original image.tar from these forums or from the samfirmware.com site and flash to be back to original untouched phone.
Hope this helps.
Ps, coming from an iPhone myself I promise this is no more complicated once you jump in
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