For questions 1 + 2: No you don't have to edit it if you are using existing ones but you have to be careful to make sure that the ones you choose are obselete. It is easier to add your own ones especially with the use of TSL Patcher.
For question 3 I am not entirely sure what you are getting at so I will explain it from scratch.
The script that begins
int StartingConditional()
Goes in the Script that determines availability box in K1 or Conditional #1 in K2 of the option that you ONLY WANT TO BE DISPLAYED WHEN THE CONDITION HAS BEEN MET.
You will need to use a script like
SetGlobalBoolean("your_boolean", TRUE);
to set the conditional therefore making the option from the above example available. What you set this to is up to you.
You can see this in the script source of my Tatooine Job Office which is all helpfully commented -
http://deadlystream.com/forum/files/file/34-tatooine-job-office/)