The poster is from CA so everything is illegal there. Here in AZ you can get denatured alcohol at any paint store or Home Depot. Water works good for brake fluid spills on the vehicle so I am guessing it would also work well to clean up a power bleeder as long as it is wiped down and dried...
I have been working on s550 brakes for about the last 7 years, so I am pretty confident. Would you please post a photo of your factory parking brake on your 2017 Mustang GT since that is the model we are discussing in this post.
Our vehicle's parking brake is mechanical cable operated, correct but it uses the caliper to apply the brake. I have seen many times where there are air bubbles that get stuck and are released by cycling the parking brake whether it is mechanical or electric.
The new fluid would mix with the old so it would actually take more fluid to flush it correctly. Also, it does not matter which caliper you start with if all the air is bled out correctly. Don't forget to cycle your parking brake when bleeding the rears to make sure all the air gets bled out.