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She had gathered these things from time to time when she had gone to the Sunday-school with the children, and had laid them up in her memory more carefully than she had done with those other words and phrases; and she had studied them deeply, for her good and ours. One may see by this that she had a wise and thoughtful head, for all there was so much lightness and vanity in it. (Mark Twain, A Dog's Tale)
Does for here mean despite? and Does lightness here mean 'light-minded'?
"for all" can be used synonymously with despite. e.g.:
For all his boasting, he's really not all that talented
It's kind of an old phrasing of the expression, but I agree with your parsing. And I think this is more like the intended usage of lightness:
lack of seriousness and stability of character often accompanied by casual heedlessness
