If you bought the car from a dealer then the easier (and cheaper) solution would be to just ask the dealer to flash it back to stock using a Tech II because you weren't expecting a car with aftermarket tune. Or if you can still contact the previous owner from a private sale you could ask if they have it.
Otherwise, I think the best way to do it and use the least number of credits (2 credits for 1 G8) would be to read the current tune and then open one of the stock tunes you downloaded as a compare file against the tune currently in the car. You could then use the HPTuners copy differences feature to copy all your settings over.
I don't think this would be undetectable by the dealer though. Reason being is I had an unlocked tune I created from my stock tune using HPTuners at the track once. I was having an issue, so I decided to let another G8 owner put his Cortex tune in my car. The tune wouldn't transfer from the Cortex handheld until I wrote my 100% stock file back to the car, then it worked fine.
If you only concerned with having the car with stock settings for your wife (and not about the warranty), this method would work fine. You would need to make sure the Calibration VCM OS ID and TCM OS ID's match in the Calibration Details view from HPTuners. Also, to be safe, the controller segments for the ECM and TCM should probably match between your current tune and the stock file you downloaded as well. Someone who is more of an expert about this may be able to confirm for sure if certain OS's and segments are cross compatible, but I can only say that it will be fine if everything matches. Also, if something doesn't match, even though it may work fine, that's another thing the dealer could notice that would tell them you were tuned at some point.
One specific problem I can think of with trying to write one of the downloaded files to your car is you will need to change the VIN # to match, so you'd have to license the downloaded file as well as your car before you could save the downloaded tune with your VIN and write it to your car. Essentially, you'll use double the amount of credits and think of it this way, every credit you use makes the tuning interface worth $50 less when you try to resell it, up to the 8 credits the interface comes with. If you only license 1 G8, then you can expect to only lose about $100ish in value when trying to resell. I say $100ish because the reality is it's worth a little less even since the product is used after all and probably doesn't have warranty for the 2nd owner.
