> The opportunity cost of time is decreasing over income, since the marginal utility of income is also decreasing, and these things should be equal in equilibrium.
In a two-good, work-leisure market, the opportunity cost of non-work time is the foregone income. It's not (just) equal at equilibrium; it's equal by definition. The two things that are equal at equilibrium (and only at equilibrium) are the marginal utility of the income and the opportunity cost of working (that is, the marginal value of an extra unit of non-work time). That's not the opportunity cost of the non-work time, which is what we're talking about when we refer to the opportunity cost of time.
> But the value of an additional dollar isn't the same for these two workers, since it's also a function of their present income.
You're confusing the marginal utility with the opportunity cost. There's a function that relates these two concepts, yes, but they're not the same thing, as you can see here:
> Richer workers can afford higher rents because their wages are higher. But that still doesn't mean that their marginal utility of income is higher than thay of lower paid workers.
Nobody said that the marginal utility of income for the higher-paid workers is greater than the marginal utility of income for lower-paid workers. In fact, that's a comparison that can't really be made - utility is explicitly not comparable between two individuals (that's a fairly fundamental axiom of microeconomics). We can say that, as an individual's income increases, the marginal impact of further increases in income decreases, but the comparison you're trying to imply here is a value judgment between two individuals.
However, we can say that the opportunity cost is higher for the higher-paid workers, because that is something that is comparable across individuals - it's dollar-denominated. The opportunity cost of non-work time for both sets of workers is the foregone income, and that will be higher for higher-paid workers (by definition). That does not diminish the importance of what either set of worker might choose to do with an additional $50 (or any fixed lump sum of money), but that's a value judgment and is not the same as the opportunity cost of the work-leisure trade-off (which is an objective measure).
> I won't touch the issue of having children, because I don't think anyone has quite yet successfully tackled human reproduction.
It's pretty well-established that children are inferior goods (in the economic sense - the income-elasticity of demand is less than zero).
> It's somewhat unintuitive, because often people go through Econ 101 without trying to understand what a decreasing marginal utility of income means
I have a degree in economics. I know how diminishing marginal utility works.