1. It's quite hard. You'll need.
a. Find an unused TAG;
b. Look in the Modding Documentation the correct ID ranges for that TAG;
c. Add the national core provinces to that TAG in the revolt.txt;
d. Add tech teams, ministers and leaders files to the proper folders using that ID ranges;
e. Add the name of these files you've created to the teams, ministers and leaders.txt files.
f. If you want to add that country not only as a releasable country you'll have to create an .inc file and edit the proper scenario...
2. Don't have idea. Maybe it's better to prevent that peace event from firing, or edit it.
3. You can't. It will change only if you change the TAG of that country.