Q1) I get +1 Pain even if I have no cards in District 13 and the High School?
Q2) What does the sentence in the parenthesis mean?
More clarity can be found on page 7 of the rules, "While the Devilish Advocate card is in the Bazaar, your total Pain in the High School and District 13 are both increased by 1 when each of those Districts is scored. However, this does not count as having a card played in either District for scoring purposes."
After scoring the High School, count the number of cards held in the High School by the player with the most cards played there. The Devilish Advocate does not count toward that sum.
District 13 awards Candles and other benefits based on the conditions specified on the Law card for that turn. Usually a reward for having the highest total Pain value on cards played into District 13. The Devilish Advocate counts toward gaining those rewards.
However, sometimes there are rewards for all players who play in District 13 (Have Us A Corpus, The Law of Averageness, No Child Left Bedside). The Devilish Advocate does not grant such rewards because it does not count as having played a card in District 13.