It doesn't exactly void the warranty, and reversing the coding options is extremely easy. Coding, is kind of like editing config files for the software, it doesn't actually change the software that's running.
4. What % of charge do you start activating this "Hold Charge Feature"
My "goal" is not to burn gasoline, so usually I drive until the REx automatically engages at 6.5-8%. If you plan on taking an extended road trip, this isn't a good idea as it leaves you with a fairly small battery buffer, which can be disastrous if some planning goes wrong. On my 350mi trip, I engaged the REx as soon as it will let me (75% SOC), in practice this means about 20-25 minutes into my trip. This gives me 75% of the battery as a buffer to work with, if I need to climb hills, get to the next gas station after the tank ran dry, whatever. From that point out it's stopping every hour or so to put another 2.3 gallons into the tank, getting a snack or restroom break if necessary. Do note that the REx may or may not provide enough power output to completely power the car, so a trip that started out at 75% when you started the REx may have had several fuel stops and a battery that is sitting closer to 60%. We drove at least the speed limit which is 80MPH most of the way, and while the car is comfortable at that speed, you will be eating away at your battery buffer at that speed.
The other case is if I'm taking a medium range trip with a mix of highway and city driving, I try to use the REx on the highway instead of in the city. I also know that if I'm going to have to use REx power to get home, and I haven't climbed the hill (read mountain) to get to my house yet, I will set hold state of charge to 15%, so that way I can climb the hill without entering reduced power mode.