Looks like ~29% of households are dual-income. Meaning we would need to adjust the mean income and contributions for the "US Household" columns by a factor of 1.29. The contributions/income stay the same though.

There's really not a good on-the-fly adjustment one could make for the median measurements. The best approximation I could come up with was 15%, (which is the national median househould income divided by the national median individual income).

I have updated the original post with these adjusted numbers. It looks like my hypothesis was incorrect. An average LWer makes less and donate less than the average American.

Note: I know this is not a perfect method for transforming household income into individual income. But after two iterations, the end result is much the same, and I really don't like the idea of creating ad hoc methodologies just because I don't like the results.