Well "ladre" could be robber though I doubt "ladraria" would have been some robber's cave but you forgot "prendriatz ma maladia" (you'd catch my disease)-with an "r" after the "d" btw. Besides, the lyrics say "siáu filha d'un ladrièr" not "ladre" which is only in the title. So... who gave the title to the song and when? Even if the lyrics were established in the early 1800's "ladre" would mean more or less the same as now = leper or miser, the language hasn't changed much. There's no linguistical clue to help. It'd be interesting to have the lyrics in their original form (this one has a lightly different spelling for some words)

About a bèu chivalièr it's funny how literature paints a nice picture of them but they were first guys who could afford a war horse and while the Northern form of the word (chevalier, chivalier...) tells about "l'esprit chevaleresque", the Southern form of it (cavalier) tells about "des façons cavalières" which are as far as I know pretty coarse and rude manners! My take is than in real life, the Southern form was more accurate!