First, I agree with your point b) as w increases, income as well as purchasing power increases. Since leisure, by assumption, is a normal good, as income goes up, the desire for more leisure should also go up.
I think of the endowment effect is that we are endowed with a fixed 24 hours per day, and there is only work or leisure. The more one works the higher the marginal value of an hour of leisure. So as hours worked goes up, so does the opportunity cost of work.
I hope this helps.