I've moved on to Chapter 17 and there is a further explanation about the use of imperfective/perfective with (по)нравнться . It says, "In the past tense the perfective is used most often, as the use of the imperfective past indicates that a person used to like something or someone, but now has changed his/her mind and no longer likes it/him/her/them." I guess that's restating what you (iCake) say under "third of all". I think I get it now.
Z