To answer the question about the stars next to leves when you trade them in, it is not based completely off of the level of the leve or the difficulty you did it at although those may be a contributing factor.
I have done leves at many different levels and difficulty within the same reset period, and there seemed to be no correlation between those two factors. In fact, some leves would give different star values when the option comes up to trade them in for a different leve. (I.E. One leve might be 1 star for the first leve I try to trade it in for, but when I go to trade it in for my second leve, it's suddenly 2 stars.) I believe level, difficulty, your score during the leve, and the type of leve all factor together in a compatibility rating that effects what type of bonus that leve gives when trading it in.
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