In response to your questions:
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EARNWT>0 is not a good way of identifying individuals with earnings data. In fact, all civilians age 16+ in outgoing rotation groups have EARNWT>0, but only those who are currently employed (and not self employed) have data from the earner study. The variable ELIGORG should instead be used to identify those with earnings data. Note that this variable does not always line up perfectly with the stated universe for the earner study, but in 2017-2019 there are very few cases of mismatch.
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The only reason you would need to divide the weights by the number of pooled samples is if you wanted to estimate population totals (e.g. the number of employed people who were in a union in a given year). For calculating averages or proportions, you don’t need to divide the weights.