With piacere subject and object change places, so, although I is the subject in English, mi is the object in Italian, and that makes lei/ella/essa (=she/it) the subject. And the subject doesn't have to be included in the sentence, because it is indicated by the verb, in this case è, which makes it 3rd person singular.
EDIT: And one more thing. Lo/la are directs object pronouns, but if we wanted to use a pronoun it would be a female subject pronoun and that's why I wrote lei/ella (for women) or essa (for animals and objects).
Probably never -- it's a clunky translation. "I liked it less" is an equally correct translation, and far more idiomatic English.
I can come up with a context for "I have liked it less" -- something like "I'm never sure whether I like spinach: at some times in my life I have liked it more, and sometimes I have liked it less" -- but it's a bit of a stretch. And even then, I suspect that the imperfect ("mi piaceva di meno") would be a more appropriate translation in that context.
No, unfortunately I have no idea. But, whatever word you use to understand the grammar structure (I use my native language), I think it's still best to convert it to like because it's probably the best translation in terms of word usage and the most natural way to say it in English. It might be more difficult at the beginnig, but will become more natural once you get used to it ;)
DL also suggests "I have liked it less" as a correct translation, but isn't that "Ho piaciuto di meno"? I see that it means the same, but I often wish DL would be more consistent and focus on the issue at hand in each lesson, unless it's an idiomatic phrase, in which case it should be noted.