Why kernel tutorials always say they need nasm. Gcc already comes with an assembler so simply use it instead of installing other softwares.
And if you prefer intel syntax simply add ".intel_syntax;"
No, a ret instruction would probably segfault, depending on the content of the stack. To terminate a program you have to use the corresponding system call. On linux :
Yes they are !
Global variables have static storage duration and are therefore default initialized.
Be careful with the word 'static' which does not always correspond to the the keyword static which has several meaning ! When used with a global variable the static keyword has not the same meaning as static storage duration". It only means no external linkage.
Don't bother having a external GUI, just use the built-in Ncruses one.