Absolutely agreed that factor 2 that I mentioned might be insubstantial, but I felt the need to mention it just in case it ended up being greater than I expected. My intuition on this issue is somewhat different than yours, and my guess is that the two largest factors (remittances and direct effects on Nigeria through less nurses) are going to roughly balance out, and that it's going to depend on the other issues which I placed under factor 4. 

You mention two of the indirect (factor 4) impacts I was thinking of, but there's definitely a lot of that kind of impact which is difficult to measure.

On its own terms as you discuss it, I absolutely agree that the original article is flawed. It's certainly the case that the issue nowhere near as straightforward as the paper's authors would have you believe. However, the question of the overall cost-benefit is also an important one, and also worth examining. I'm going to start working on a basic model, and I'll post here (and maybe in a separate post as well) once I've completed it to a level I'm content with. The overall issue at work here, of brain drain vs. remittances appears to me to be a very important issue in  global development, with this being an example where the costs of brain drain appears higher than usual. If the effect of remittances is powerful enough to outweigh brain drain even in this case, it could have broader impacts in terms of immigration as a net QALY increase. 

There already does appear to be some evidence towards an effect of "brain gain" where even medical immigration appears to improve net wellbeing through remittances, as seen in the example of the Philippines here, but it's a complex issue, and the medical situation in the Philippines is different from the situation in Nigeria.

This is a good point about the CGD paper I believe, though to some extent it's based on whether to look at overall flows rather than flows specifically to the UK. I think you make a strong case that it's best to look at overall flows, and show issues with the WHO per capita data. However, I don't think this gets anywhere near actually solving whether or not this can be a "win-win" for both countries. It is easily possible that nurse emigration, despite lowering the supply of nurses in Nigeria, can still be a win-win due to remittances. There is abundant evidence that remittances promote financial development and long-term economic growth. In order to determine whether or not this emmigration is a net benefit for both countries, one would have to take into account:

1. The benefit that the nurse gives in Nigeria.
2. The benefit the nurse gives in the UK.
3. The benefit to Nigeria from remittances the nurse sends back.
4. Any long-term effects that might come from having talented Nigerians leave for the 
   UK, and long-term effects a nurse shortage could have in Nigeria. 

To me, this seems like a complicated question that would take many hours of research to reach a  satisfactory conclusion to.

How do you square this with the experience of J.C. Penney, who, after eliminating sales and instead choosing to offer "everyday low prices", experienced a 20% drop in sales? It's certainly tempting to believe that discounts and other such unnecessary promotions don't work, but it seems that sales do to some extent stimulate, well, sales.

See https://hbr.org/2012/05/can-there-ever-be-a-fair-price.