Your question is still useful for anyone else reading this.
1. Modem is personal choice. Also remember modem does not equal bootloader, it is just a way to access phone service and data. I am currently using MK5, MG2 was solid as well, and I have used the MK3 from Sprint no problem.
2. SELinux was introduced in 4.3, and was set to "Permissive" by default. In 4.4, the default is "Enforcing," which can mess with some root apps and their functionality unless it is manually set back to "Permissive." Think of it as Google's very light version of Knox, I can't think of any other comparison right now. My attempt to modify build.prop didn't stick, and I think this is an intentional design issue to prevent direct modification, as I have read about others having the same issue. To manually change: Open Terminal Emulator, and give it root access with the command "su" (don't use quotes), followed by the enter key. On the next line you will see it starts with # instead of $, this means root has been granted. On that next line, type "setenforce 0" (again, no quotes, and that is a zero not the letter O; this is just basic 0 or 1 meaning true or false in computer-land) and then hit enter key again. Make sure to close the program entirely after that or it will run in the background and consume battery. This has to be done after every reboot due to "Enforcing" being part of the build.prop.
SELinux has only given me problems a couple of times, so this is just more of a nerd thing than anything else, but I always feel there is no such thing as too much knowledge.
